X RESriRATORY ORGANS '279 



gill-less.^ The spiracle is a vestigial cleft. At an early stage 

 of embryonic growth it differs but little from its fellows, but 

 subsequently degenerating it is represented in the adult by a 

 tubular passage between the oral cavity and the exterior, which, 

 however, is often complicated by the development of caecal out- 

 growths.^ The anterior wall of the spiracle often retains a 

 rudiment of a hemibranch in the shape of more or fewer vascular 

 lamellae, which, as they are supplied with arterial blood, and not 

 with venous blood like the ordinary gills, are said to form a man- 

 dibular or spiracular " pseudobranch." The spiracle varies greatly 

 in size in different families, being largest in the Trygons and 

 Torpedos, and very small, or even absent in the Lamnidae. Its 

 pseudobranch is best developed in the Notidanidae, where it has 

 the essential structure of a true hemibranch, and, as in other 

 Elasmobranchs, but to a greater extent, probably aids in the 

 additional aeration of the blood which is distributed to the eye 

 and brain. The characteristic opercular covering of the external 

 apertures of the gill-clefts in the Teleostomi and Dipnoi is 

 wanting in Elasmobranchs. It is interesting to note, however, 

 that in CJilamydoselachus ^ curious frilled cutaneous folds are 

 developed as extensions of the outer edges of the inter-branchial 

 septa, as well as of the hyoid region, and, like a series of 

 incipient opercula, project backwards over the successive branchial 

 clefts (Tig. 252). 



While in many respects more primitive than in Elasmobranchs 

 the branchial system of the Cyclostomata presents certain special 

 and peculiar features. The branchial clefts assume the form of 

 oval, antero-posteriorly flattened pouches or sacs, varying, how- 

 ever, in number, and in their mode of communicating with 

 the exterior, in diiferent genera. In the Lamprey (Fetro- 

 myzon) there are seven pairs of obliquely- disposed gill-sacs 

 opening externally by small rounded orifices, and by similar 

 apertures, not directly into the pharynx, but into a branchial 

 canal (Fig. 162, r.t), which underlies the oesophagus, and, while 

 ending blindly behind' the last pair of sacs, communicating in 



^ In those Elasmobranchs which have more than five branchial clefts there is a 

 correspoilding increase in the number of branchial arches and hemibranchs. 



- Ridewood, Anat. Anz. xi. 1895, p. 425. 



' Garman, Bull. Mus. Gomp. Zool. Harvard, xii. 1885, p. 1 ; GUnther, Challenger 

 Ileports, "Zool." xxii. 1887, p. 2. 



