VERTEBRATES. 43 



The large intestine is shorter, as a rule, in the lower than in the liigher forms, and 

 has little significance in the absorption of food. It may terminate independently on 

 the surface by a separate anal aperture, or may open into an involuted area of the skin 

 (the cloaca), into which the ducts of the urinary and genital organs also empty. 



THE RESPIRATORY ORGANS. 



The Gills. 



It has been indicated above (p. 15) that all vertebrates have the anterior part of the 

 .alimentary canal jierforated by visceral clefts, which primitively .allow streams of 

 water to flow over the gills formed on their walls. The clefts originate as hollow out- 

 growths of the alimentary canal, which extend to the skin, which is subsequently per- 

 forated. In air-breathing forms the clefts are only present in the embryo, and 

 disappear entirely or undergo a change of function in the adult. They are much more 

 numerous in Amphioxus, and even in the Myzonts, than in the more typical fishes, 

 where they are ordinarily six in number. Of these, the first is called hyomandibular, 

 as it is situated between the mandibular and hyoid arches ; the others are generally 

 termed branchial clefts, and are sep.arated from each other by more or less complete 

 partitions bearing the gill-filaments, and strengthened by the br-uichial arches described 

 above. The partitions referred to are comjilete in the sharks, where the clefts open 

 independently on the outside by a series of slits, but in the Ganoids and Teleosts the 

 partitions are very rudimentary, the branchial arches carrying little else than the gill- 

 filaments and the vessels and nerves distributed to them. In this way the gill-clefts 

 are extremely wide in the Teleosts, but they are not visible on the sides of the head as 

 in the sharks, for they are concealed by the gill-cover, a fold of skin which projects 

 back from the hyoid ai'ch, and is strengthened by the opercular bones. This gill-cover 

 not only serves to protect the gill-filaments, which would otherwise be exposed, but 

 aids, by its movements, in promoting a stream of fresh water over theii- surfaces. Each 

 gill-bearing arch, except the first and last, bears two rows of gill-filaments, one of 

 which looks into the pouch or cleft in front, the other into that behind it, but there is 

 hardly any trace of a partition separating the rows in the Teleosts, so that the two 

 series of filaments count as one gill. 



The hyomandibular cleft loses its respiratory function very early in the vertebrate 

 series, and either disappears or persists with an altered function, as was described under 

 the auditory apparatus. It is present, more or less completely, in many sharks and 

 Ganoids, and frequently opens to the outside by an aperture behind the eye, — the 

 spiracle. From this opening, the cleft is sometimes known as the spiracular cleft, and 

 a rudimentary gill situated on its anterior wall as the spiracular gill; but this gill is 

 not functional, — it receives only already aerated blood, and is therefore known as a false 

 gill or pseudobranch. The hyoid arch carries only one row of gill filaments (the poste- 

 rior) v/liich help in the aeration of the blood in Sharks and some Ganoids, but receive 

 only blood already aerated in the Teleosts, and consequently constitute a hyoid or oper- 

 cular pseudobranch. 



Two very different classes of structures are known as external gills. The embryos 

 of many Selachians possess very long filaments, which extend out from the pouches to 

 the outside (Fig. 16) and apparently have a nutritive function, their epithelial cover- 

 ing serving to absorb yolk particles; but the nuid-])up|)y (Nectnrits) and similar peren- 

 nibranchiate Amphibia possess permanent, other forms only temporary, external gills, 



