x RESPIRATORY 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- 
erowths.” 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 Chlamydoselachus* 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 (Fig. 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 different genera. In the Lamprey (Petro- 
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, 7.t), which underlies the oesophagus, and, while 
ending blindly behind the last pair of sacs, communicating in 
1 In those Elasmobranchs which have more than five branchial clefts there is a 
corresponding increase in the number of branchial arches and hemibranchs. 
* Ridewood, Anat. Anz. xi. 1895, p. 425. 
3 Garman, Bull. Mus. Comp. Zool. Harvard, xii. 1885, p. 1; Giinther, Challenger 
Reports, ‘* Zool.” xxii. 1887, p. 2. 
