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 Chatopoda ; 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 1 glands, 

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

 tubes they inhabit. In Aphrodite the dorsal setiparous glands produce 

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

 chambers. The setiparous glands may again become simple dermal 

 glands. Anackcda, for example, no longer possesses setae, but, in place 

 of the 4 rows of setae of the related Enchytmus 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, Hirudinea). 



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 Gordiidcr 

 among the Nemathelmia 

 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- 

 cular finely granulated 

 layer containing scattered 

 nuclei. In this reduced 

 form we meet with the 

 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 Ilirudinea and in most Oli/jocha'ta, as sexual maturity begins, 

 the hypodermis undergoes a peculiar metamorphosis in a series of the 

 segments near the genital apertures (in the Hirudinea ahvays 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 clitellum). 

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



FIG. 120. Transverse section through the middle part 

 of the body of a Nemertian, half diagrammatic. In, Lateral 

 longitudinal nerves ; <lii, medio dorsal nerve ; Inn, basal mem- 

 brane ; rm,' circular muscle layer; Im, longitudinal muscle 

 layer; rs, proboscis sheath ; /, proboscis ; ?<?, dorsal vessel ; 

 vl, lateral vessels ; h, testes ; p, parenchyma ; tnd, mid-^ut. 



