CHAPTER IX 

 SCYLLIUM CANICULA (continued) 



Alimentary system Respiratory system Circulatory system 

 Urogenital system. 



Alimentary System. 



The digestive system consists of a long canal running from 

 mouth to anus, mainly in the coelom, and it is about twice as long 

 as the distance between these two points. Certain glands are 

 connected with this canal as in the frog, and they play important 

 roles in the digestive processes. 



The crescentic mouth leads into a fairly capacious oral or buccal 

 cavity whose walls possess no salivary glands, and in the floor of 

 which the basihyal cartilage forms a low projection, having almost 

 the appearance of a tongue, although it cannot be considered as such 

 an organ. The whole cavity, being developed as a stomodoeum, is 

 lined with ectoderm, and owing to this fact is able to bear the teeth. 

 It has been pointed out previously that the teeth are homologous 

 with the denticles covering the skin, and, indeed, in the embryo 

 they are in a continuous sheet with them, not being separated until 

 later by the formation of a scaleless lip groove. The teeth are borne 

 in a series of parallel lines over the jaws, in which they are not 

 embedded, but to which their bases are attached by a very tough 

 fibrous tissue. As all are similar in structure, and not different from 

 one another like our own, the dentition is said to be homodont. Also, 

 since there are a number of rows, indeed, as fast as the outer row 

 wears out a new row takes its place, we find a number of successions 

 of teeth, a condition termed polyphyodont. This stands in marked 

 contrast to ourselves, where but two sets are fully developed, the 

 milk teeth and the permanent teeth, that is, a diphyodont condition. 



The hinder end of the roof of the buccal cavity bears a small 

 transverse fold of skin, the oral valve, otherwise the cavity passes 

 over imperceptibly into the pharynx. There is a noteworthy 

 morphological difference between the two regions, however, since 

 the pharynx is lined by entoderm and constitutes the beginning of 

 the mid-gut or mesenteron. The pharynx is a fairly short flat tube 

 for the passage of the food, but being mainly concerned with the 

 function of respiration, and its ventro-lateral walls are marked by 



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