IV 



VERMESTHE INTEG UMEXT 



191 



B 



are joined by a hinge is dorsal in the mussels, in the Brachiopoda it 

 is posterior. 



We have already, in 

 the systematic review, said 

 what is most important con- 

 cerning the oral tentacles 

 and oral arms, which are 

 very characteristic of the 

 Prosopygia the various 

 positions of the anus were 

 also described. Refer 

 to it also for the outer 

 form and organisation of 

 the Piotatoria and CJuetog- 

 natha. 



FIG. 125. Rhynconella psittacea. 

 A, From above. B, From the left side. 



II. The Integument. 



The integument of the worms consists first of the outer cuticle, and 

 second of the subjacent body epithelium, which secretes the cuticle, and 

 which in most worms (just as in the ArtJvropoda) is called the hypodermis. 



The cuticle is very variously developed. It is thin and delicate in 

 the soft-bodied forms, especially in the Nemertina, where it is perforated 

 by very fine pores to allow the cilia to pass through. Where it is 

 strongly developed, as in many Annulata, Prosopygia, Nematoda, and 

 Piotatoria, it gives protection and support to the body, and as skeleton 

 offers support and surfaces of attachment to the body musculature. It 

 consists of a substance allied to chitin, and occasionally calcifies (in 

 Bryozoa, Brachiopoda) into a very hard envelope or shell. It often 

 shows stratification, and seems to be composed of various crossing 

 systems of very fine adhering fibrillce. It may in general be conceived 

 of as a secretory product of the glandular hypodermis cells which 

 underlie it, or as a product of metabolism of the protoplasm of these 

 cells. To the same category as the cuticle belong various sorts of 

 tubes and envelopes which, detached from the integument, surround 

 the bodies of many Annelida (tube-worms) and possess a chitinous 

 substratum. We must further consider the setae of the Chcetppoda 

 as cuticular formations of certain hypodermal glands, which can be 

 seen, at least when they first appear, to be composed of fibrillae and 

 fibres in close contact and glued together. 



The cellular hypodermis, which usually consists of one layer, can 

 best be examined in the Annulata, where it is composed of the two 

 following principal elements (1) gland cells ; these are naked and 

 large, and yield the material for cuticular formations ; over each 

 gland cell there is generally a pore in the cuticle ; (2) thread-like 

 cells ; these are generally slender cells whose protoplasm is strongly 

 modified and falls into fibres. They often lose their nuclei, and 

 are arranged round the gland cells in such a way as to form for the 



