190 COMPARATIVE ANATOMY CHAP. 



poda is covered with a chitinous cuticle which is specially strongly 

 developed in the Polyclueta Errantia. The cuticle is weaker and 

 much more delicate in most of the Oligochceta which live in mud, 

 and in the tubicolous Polychceta. 



For the general form of body and outer organisation of the Myzo- 

 stomidce compare the systematic review. 



The form of the body in the Prosopygia is .extremely varied. The 

 most important points have already been referred to in the systematic 

 review. The body is as little segmented externally as internally. 



In the Sipunculacea (Sipunculus, Priapulus, Halicryptus} a regular 

 outward ringing of the trunk occurs. The rings, at least in some 

 cases, correspond with the muscle bundles of the circular musculature 

 and with the lateral nerves which proceed from the ventral strand. 

 In Sipunculus there are, in addition to the circular furrows on the 

 trunk, still deeper and more distinct longitudinal furrows, so that the 

 Avhole skin seems divided into regular die-shaped areas. There are 

 similar longitudinal furrows on the "proboscis" of Priapulus. Papilla? 

 are very wide spread on the bodies of the Sipunculacea, principally on 

 the proboscis. 



A closer comparison of the proboscis of the Sipunculacea with the 

 similarly named organ of the Echiuridce (which formerly were united 

 with them in the class of the Gephyrea) shows great morphological 

 difference between the two organs. The proboscis of the Sipun- 

 culacea is the front portion of the body, which can be invaginated 

 into the hinder portion. The mouth lies at its anterior end. The 

 proboscis of the Echiuridce is a prolongation of the head portion 

 (prostomium), which lies in front of and above the mouth and cannot 

 be invaginated. The mouth lies at its base. In the proboscis of the 

 Sipunculacea runs the fore-gut, while the fore-gut is in no way con- 

 nected with the prostomium of the Echiuridce. 



The Sipunculacea possess a rough cuticle ; in Phoronis it is delicate, 

 and the skin therefore secretes a detached chitinous envelope, which 

 serves as a dwelling tube. The Bryozoa generally form a rough hard 

 cuticle (cell, ectocyst) whose aperture can be closed by a cover, and 

 which often calcifies. In a similar way the mantle of the Brachiopoda 

 secretes a bivalve shell which is generally calcareous, less frequently 

 horny. 



This shell of the Brachiopoda (Fig. 125) cannot be compared 

 with the similarly bivalved shell of the Mussel (Lamellibranchiata, 

 Cochlidce). The two shell valves of the former are dorsal and ventral ; 

 each valve is symmetrical ; the median plane of the body divides 

 each valve into two lateral congruent halves. In the mussels, on 

 the contrary, we distinguish a right and a left shell. The median 

 plane passes between the two shell valves. Each valve is asym- 

 metrical. The gaping edge of the shell in the mussels is ventral, in 

 the Brachiopoda anterior ; the closed edge where the two valves 



