280 Bashford Dean Memorial Volume 



GILL OPENINGS OF THE FRILLED SHARK 



Although in most sharks five pairs of gill-openings are present, in Chlamydoselachus 

 there are six. This at once places Chlamydoselachus in the same category with the other 

 primitive selachians, Hexanchus (with six gill-openings) and Heptanchus or Heptranchias 

 (with seven). 



Each gill'opening is guarded by a thin caudally projecting fold or flap which doubtless 

 serves as a valve in addition to providing protection for the delicate gill-filaments. The gill- 

 openings are very large, so that when the great mouth is open for seizing prey, water 

 entering it may readily pass out through the gill-apertures. The members of each series of 

 gill-covers overlap like the shingles of a roof, and each is supported by a number of 

 branchial rays ( Figure 6, plate II). In embryonic forms, the gill-filaments project beyond 

 the gill-covers, and even in adults the filaments tend to show beyond the free edges of the 

 gill-flaps. This we have found in some very old and much faded photographs of adult 

 specimens of Chlamydoselachus among Dean's materials. These we think he must have 

 brought from Japan. Moreover, AUis (1923) shows the ends of the gill-filaments project- 

 ing from under the gill-flaps (Figure 7, plate II). This and the other heads on which 

 Allis worked came either from Dean, or from Japan through Dean's kind services (AlUs, 

 1925, p. 123). None of our adult specimens show extended gill-filaments, so it is evident 

 that not all specimens have them. In this respect as in many others, Chlamydoselachus 

 is decidedly variable. 



The gill-covers constituting the first (the anterior or opercular) pair require special 

 consideration. These begin a little further dorsally than the others (Text-figures 7 and 8; 

 Figures 6 and 7, plate II; and Figure 16, plate V), and extend so far ventrally that they 

 meet and are continuous across the median line, forming a frill or ruffle (Text-figure 8). 

 In this respect the frilled shark differs from all others of its kind, for in no other selachian 

 does the first pair of gill-folds extend so far ventrally as to meet and form a structure whose 

 free edge is continuous across the isthmus. Thus in Chlamydoselachus alone among sharks, 

 does the first or opercular pair of gill-folds foreshadow the condition found in many 

 teleosts where "the gill-membranes are free from the isthmus." However, it may be noted 

 that in the giant basking shark, Cetorhinus maximus, the gill-clefts arise high on the 

 dorsum, as they do in Chlamydoselachus, and extend so far ventrally as almost to meet. 

 Still Garman says even of the anterior pair in Cetorhinus that they are "narrowly separated 

 on the back and on the throat." In Chlamydoselachus the gill-covers posterior to the first 

 pair fail to meet ventrally, although their ends approach closer on this surface than on the 

 dorsum — as may be seen in Text-figure 8. 



We have noted that Chlamydoselachus has an extremely large mouth correlated with 

 voluminous gill-openings. Similar structures are found in the basking shark, Cetorhinus 

 maximus, which feeds on small pelagic organisms caught with its large mouth while 

 it is swimming at or near the surface. The whale shark, Rhineodon typus, also is known to 

 feed on such food, and it has a mouth which may correctly be characterized as cavernous; 

 its gill-openings, too, are unusually large. But any inferences regarding the feeding habits 



