Notes on the Anatomy of Cacopus systoma, an Indian Toad of the Family Engystomatidæ.
- 1In the pharynx a new structure is situated to which the status of an organ may be given, and it has been accordingly named “the pharyngeal organ of Cacopus systoma.”
- 2Sub-cutaneous RS well as sub-peritoneal fat-bodies are normally present in this toad; while the former are developed in relation to the lymphatic septa, the latter are probably deposited in connection with the peritoneal membrane. Their function, as has been ascertained, is to assist in the general metabolism of the body during æstivation.
- 3The stoutness of this toad is, in the first instance, due to the enormously developed viscera. This condition is improved upon by the inclusion of a part of the thigh into the contour of the trunk and by the enlarged sub-cutaneous lymph-sinuses.
- 4In the skeleton the vestigial state of several of the bones of the head is remarkable. This and the consequent engystomatous mouth are, in my opinion, related in some unknown manner to the nature of the diet, which consists almost entirely of termites*, winged or wingless according to the season. Witness, in this connection, the similar effect produced by a similar diet on the Myrmecophagidæ among Mammals.
- 5It is noteworthy that the pre-hallux of Rana, usually regarded as a degenerate digit, has in this toad become an organ with a definite function, viz. the so-called “inner metatarsal tubercle.”
- 6While certain muscles, for example the sterno-radialis and the deltoideus, show a remarkable degeneration, other muscles, for instance the muscles of the hyoid and of the floor of the mouth, exhibit an equally remarkable specialisation. The muscles examined also show important differences in their relative dimensions, origins, and insertions. Certain new muscles are present which have been already described, while three muscles are absent, viz. the cutaneus-pectoris, omo-hyoid, and the rectus-femoris-anticus. Again, certain non-cutaneous muscles of Rana have acquired a partial connection with the skin, for example, the rectus-internus and the portio-abdominis. Assuming the muscular anatomy of Rana to be typical of the Anura, one may trace the differences in the musculature of this toad to two factors, viz. the fossorial habit and the nature of its food. This creature passes the greater part of its life underground, seeking water only for heeding purposes. This implies that, while the same muscles called into frequent activity in an aquatic frog may suffer by the want of regular use, other muscles of a direct utility in burrowing are likely to be improved anti exaggerated. At the same time, the curiously specialised muscles of the hyoid and of the floor ot the mouth probably indicate a more efficient method of capturing insects.
- 7Beddard (1) mentions three characters which may be common to the Engystomatidæ: these are (1) the partially cutaneous origin of the rectus-internus-minor, the specialisation of the muscles of the floor of the mouth, and the division of the “outer portion of the sternohyoideus.” One may say that in these respects Cacopus systoma also agrees with the three genera described by him. Nevertheless, in the details of the second feature mentioned there is hardly any striking resemblance. I should add that the omo-hyoid muscle is generally absent among the same family, as it has not been found either in this toad or in Breviceps and Hemisus.