192 



COMPARATIVE ANATOMY 



CHAP. 



latter a loose supporting tissue, in whose meshes the gland cells lie. 

 Gland cells of the hypodermis form the chief component part of the 

 subdermal setiparous sacs of the Chcetopoda; these are the setiparous 

 glands which produce the setae. 



The setiparous glands may undergo important transformations. 

 In Polyodontes (Aphroditidce), for example, the setiparous glands of the 

 dorsal branch of the parapodia are changed into large spinning' glands, 

 whose thread-like secretion yields the material for the structure of the 

 tubes they inhabit. In Aphrodite the dorsal setiparous glands produce 

 setse and hairs, which form the hairy felt covering the respiratory 

 chambers. The setiparous glands may again become simple dermal 

 glands. Anachwta, for example, no longer possesses setse, but, in place 

 of the 4 rows of setaj of the related Enchytrceus species, has 4 rows of 

 flask-shaped hypodermal glands projecting into the body cavity. The 

 mucous glands are peculiar dermal glands which are common, especially 

 in naked and soft worms {Nemertina, Hinidinea). 



The hypodermis may 

 be very insignificant in 

 comparison with the 

 cuticle. Its elements 

 may fuse into a sub- 

 cuticular layer of proto- 

 plasm. In the Gordiidce 

 among the Nemathehnia 

 we still find it clearly 

 developed into an epi- 

 thelial layer at the an- 

 terior and posterior ends 

 of the body, while in the 

 rest of the body it is 

 reduced to a subcuti- 

 granulated 



cular finely 



Fig. 126.— Transverse section througli tlie middle part . . - 



of tlie body of a Nemertian, half diagrammatic. In, Lateral layer containing scattered 



longitudinal nerves ; dii, medio dorsal nerve ; Siii, basal mem- nUclei In this reduced 



brane ; rm,* circular muscle layer ; Im, longitudinal muscle _p * ■ r, i 



layer ; 7's, proboscis sheatli ; r, proboscis ; vd, dorsal vessel ; J^Orm We meet 'Wltn the 



vl, lateral vessels ; h, testes ; p, parenchyma ; md, mid-gut. hypodermis in all other 



Nemathelmia, where it is 

 often hardly recognisable. It seems here to be almost entirely taken 

 up in the formation of the strong cuticle. The same is the case in the 

 Bryozoa. 



In the Hirudinea and in most Oligochceta, as sexual maturity begins, 

 the hypodermis undergoes a peculiar metamorphosis in a series of the 

 segments near the genital apertures (in the Eirudinea always the 

 tenth, eleventh, and twelfth.) The gland cells here swell greatly and 

 come to lie in several superimposed layers, and there thus arises a girdle- 

 like thickening of the body which is outwardly visible (the elitellum). 



In the gill-less Annulata (higher Oligochceta, Hirudinea) capillaries of 



