154 



THE ELASMOBRANCH FISHES 



dorsal constrictor muscle. A similar valve, although less well developed, 

 occurs in some of the sharks, as for example in Acanthias and Musteliis. 



The internal branchial apertures of the branchial pockets may be simple 

 slits or the aperture may be modified by the presence of certain gill rakers. 

 The gill rakers in Squalus sucklii (gr., fig. 147) form a series of processes from 

 the pharjaigeal arches across the internal branchial apertures. On the first 



two branchial arches they project 

 only from the anterior surface of 

 the arch, while on the third and 

 fourth they extend from both the 

 anterior and the posterior sur- 

 faces. These projections are cov- 

 ered with caps of the mucous lin- 

 ing, on which stomodeal denticles 

 are present, and are supported 

 internally by cartilages, the ante- 

 rior ones of which practically 

 touch the internal branchial arch 

 and the posterior ones are directed outward from the adductor muscle (ad.). 

 In Cetorhinus a straining apparatus is differently formed. As we have 

 remarked (p. 37) the gill rakers in Cetorhinus and Rhinodon are modifica- 

 tions of placoid scales which arise from semilunar bases and are continued as 

 long filaments across the internal branchial aperture. Upon opening a gill 

 pocket in Cetorhinus (fig. 148a) myriads of these rakers or filaments (g.r.) 

 are seen to be attached to the internal branchial arches and to extend inward 

 so that those from the arch in front of the pocket overlap those from the arch 

 behind it (fig. 148b). There is thus formed of these rakers a V-shaped strain- 



rig. 146. Transverse section through spiracles, 

 ScyUium. (From Eidewood.) 



dc, dorsal caecum. 



Fig. 147. Horizontal section cutting through the gill pockets to show gill rakers, Squalus 

 sucMii. (H. M. Gilkey, del.) 



ad., adductor muscle; af., afferent artery; h.r., branchial ray; efc.\ anterior efferent- 

 collector; gr., gill rakers; ia., internal branchial aperture; ih.m., interbranchial muscle. 



ing apparatus which points into the pharynx and completely covers the inter- 

 nal branchial aperture. By means of this strainer small organisms, prevented 

 from passing out with the respiratory current, are collected in great numbers 

 and passed down the digestive tract as food. 



