The Anatomy of Chlamydoselachus 



423 



surface is very large — perhaps larger, in proportion to body size, than in most elasmo- 

 branchs. The blood vessels of the gills are described in the section on the blood- vascular 

 system, but it may be noted here that, thin as they are, the lamellae nevertheless contain 

 exceedingly rich capillary plexuses. 



The general plan of a gill of Chlamydoselachus is much like that of Heptanchus (Text' 

 figure 81, which should be compared vv^ith Text'figure 78). Indeed, so far as the gills 

 of elasmobranchs have been studied, there is a considerable degree of uniformity in their 

 structure throughout the group. 



From my observations I conclude that the gills of Chlamydoselachus are of the usual 

 elasmo branch type. In proportion to body size, the gill-clefts are unusually long (Text- 

 figure 4); they are separated by very slender branchial arches. The widely-distensible 



Text-figure 81. 

 Section, cutting parallel to branchial filaments, through second holobranch of Heptanchus maculatus. 



ad., adductor muscle; a/., third afferent artery; b.r., branchial ray cut short; csd., fourth dorsal constrictor muscle; eb., epi- 

 branchial segment of cartilaginous branchial arch; efc.4-5, fourth and fifth efferent collector arteries; ex.b., extrabranchial 

 cartilage; fi.a., anterior filament; fi.p., posterior filament; ib.d., dorsal interbranchial muscle; n., posterior division of the 



branchial nerve. 

 After Daniel, 1934, Fig. 143. 



pharynx is adapted for the rapid expulsion of a large volume of water through the gill- 

 clefts. This, in connection with the large respiratory surface afforded by the gill-filaments 

 and particularly by their lamellae, makes an efficient mechanism for aerating the blood. 



A discussion of the question as to the phylogenetic significance of the unusually 

 large number of gill-clefts and gill-arches in Chlamydoselachus and the notidanids would 

 lead us too far afield. Considerable data regarding the number of gill-clefts, from Amphiox- 

 us through the cyclostomes and fishes to the amphibian Cryptobranchus, is presented by 

 Corrington (1930, pp. 246-251), together with a discussion of the subject from an evolu- 

 tionary point of view. 



THE SPIRACLES 



The spiracles of elasmobranchs derive special interest from the fact that they arise 

 through modifications of a primitive first pair of gill-slits (Text-figure 62, p. 388) which, in 

 mammals, are represented by Eustachian tubes, tympanic cavities and external auditory 

 meatuses. In elasmobranchs the modifications are almost entirely concerned with the 

 regulation of the respiratory current, but the anatomical relations of certain parts presage 

 their use in connection with organs of hearing. 



