The present paper deals with the comparative morphology of some members of two widely spread and important families of parasitic copepods, the Chondracanthidæ and the Lernæopodidæ. The difference in their external and internal structures can be summarized as follows:—
External Features of Female.
- 1Mode of attachment to the host. It has become a rule that the antennæ, which are small and armed with simple but sometimes powerful terminal hooks, serve as the attachment-organ among the Chondracanthidæ. Subsequently the head is wholly or partially embedded in the tissues of the host. In the Lernæopodidæ there is a migration of the second maxillæ to the posterior end of the cephalothorax. They are somewhat elongated, highly muscular, partially or wholly fused, and attach themselves by means of a bulla into the tissues of the host, in order to procure a sure anchorage.
- 2The head in Chondracanthidæ is generally small and well defined from the thorax, while it is fused with the latter in Lernæopodidæ.
- 3The thorax is short, bisegmented in Chondracanthidæ, and each segment bears a pair of bipartite thoracic appendages which are more or less rudimentary. There is no thorax in the Lernæopodidæ, since in the course of development it loses appendages and segmentation, and becomes fused with the head. This results in the formation of an elongated cephalothorax.
- 4The trunk in the Chondracanthidæ is slightly flattened dorso-ventrally, divided usually into two portions by a transverse constriction, more or less distinct, but sometimes indistinct. There may be outgrowths in the form of lobes or prolongations. The trunk in the Lernæopodidæ is generally sub-cylindrical, sometimes separated by a marked groove from the cephalothorax; there are no grooves or lobes on the surface of the genital segment.
- 5The abdomen is small and unsegmented in the Chondracanthidæ, but well defined, while, if present in the Lernæopodidæ, it is one segment which is more or less fused with the trunk.
- 6Egg-strings are in both families usually short, containing numerous ova, but sometimes may be elongated and twisted.
- 7The structure of the mouth in the Chondracanthidæ shows that it functions for biting as well as sucking, and this may lead to the conclusion that the parasite feeds on a mixed diet, while it is of a pure sucking nature in the case of Lernæopodidæ.
Internal Features of the Female.
The internal features in both families are essentially the same, with slight modifications to meet the requirements of the modes of living and their method of fixation to the host. In both there is no sharp definition between stomach and intestine; there are no circulatory organs, and excretion is carried out by glands situated in the anterior part of the body, and generally in the head-region at the base of the maxillipedes and second maxillæ. The nervous system is concentrated in the anterior part of the body around the œsophagus, and is here represented by a supra-œsophageal ganglion and an infra-œsophageal ganglion which gives off a posterior nerve. The reproductive system is made up of small inconspicuous paired ovaries, large paired oviducts which fill the cavity between the alimentary canal and body-wall, paired cement-glands, and an unpaired sperm-receptacle that contained spermatozoa in all the specimens examined. This very close similarity between the internal anatomy may lead to the suggestion that these two families are derived from a common ancestry.
External Features of the Males.
The males in both families are very small in comparison with the size of females; they never become fixed in position, and hence do not usually show as much degeneration as the females. The males of the Chondracanthidæ are fixed to their females by means of strong hooked antennæ, while those of Lernæopodidæ are fixed by the second maxillæ, which retain powerful claws similar to those of the maxillipedes. The males are distinctly segmented in Chondracanthidæ, have no dorsal carapace, and bear two pairs of small and simple thoracic appendages, while in the species examined belonging to the Lernæopodidæ there is no segmentation of the trunk and no thoracic appendages. A strong dorsal carapace is present in Brachiella merluccii and Brachiella obesa, but absent in Parabrachiella insidiosa and Clavella uncinata. At the posterior end of the trunk in both families arises a pair of small abdominal appendages.
Internal Features of the Males.
The internal features of the males are apparently similar to each other in both families. The alimentary canal is at first a short œsophagus, then becomes dilated, runs along the axis of the animal, and ends bluntly before it reaches the posterior end. Gland-cells do not seem to be present. Excretory glands lie in the anterior end and generally resemble the corresponding ones in the female. The nervous system is concentrated anteriorly as in the female into a small supra-œsophageal, a large infra-œsophageal ganglion, and a posterior nerve. The reproductive system consists of a comparatively large and conspicuous pair of testes, somewhat convoluted and swollen, a pair of vasa deferentia, and an unpaired genital opening with or without a genital process, which arms the genital tube. The spermatophores are spindle-shaped in the males of all the species examined.