x PHYLUM ANNULATA 489 



The alimentary canal is not definitely divided into regions, 

 but is a thin-walled tube, the oral part known as the pharynx, a 

 slightly narrower part succeeding as the oesophagus, the greater 

 part as the intestine, and the terminal part as the rectum. A ciliated 

 groove runs throughout the length of the canal. The intestine is 

 bent on itself, and spirally twisted as it runs forwards to the anal 

 opening, which, as already noted, is situated far forwards on the 

 dorsal surface : at the junction of intestine and rectum is a single 

 simple caecum or a pair of caeca ; and a number of small branching 

 glandular appendages are attached to the rectum close to the anal 

 opening. 



There are no specialised organs of respiration in the 

 Sipunculoidea. A blood-vascular system is sometimes present, 

 sometimes absent. When present, as it is in most, it usually com- 

 prises a contractile dorsal vessel closely applied to the intestine, 

 and a peri-pharyngeal ring or plexus. 



The nervous system (Fig. 404) consists of a nerve-ring, 

 sometimes greatly elongated, surrounding the anterior part of 

 the alimentary canal, with a dorsal and anterior thickening 

 representing a cerebral ganglion ; and of a nerve-cord, devoid of 

 ganglia, running backwards from this along the middle of the 

 ventral surface, and giving off pairs of branches at regular intervals. 

 Eyes of a very simple character, consisting of mere spots of pigment, 

 are present in some. 



In the majority of cases there is a pair of nephridia or brown 

 tubes which open externally on the ventral surface, and internally 

 communicate with the ccelome by means of ciliated apertures, the 

 form and position of which vary in different cases. They act as 

 efferent ducts for the reproductive elements (gonoducts) ; but 

 their function as excretory organs has not been definitely established. 

 In many* cases one of these is absent. 



The sexes are usually distinct, and the reproductive organs 

 are of very simple character, consisting merely of ridges or clumps 

 of cells (gonads), sometimes enclosed in a membrane, developed at 

 various points on the body-wall or on the wall of one of the main 

 blood-vessels. The cells of these ovaries or testes may develop 

 in situ into perfect ova or sperms ; more usually they become 

 detached, and undergo the later stages of their development while 

 floating in the ccelomic fluid. 



The early stages of the development closely resemble those of 

 the embryo of one of the Polychseta, and a stage corresponding to 

 the trochophore of that class is developed, but with the mouth 

 situated further forward in front of the ring of cilia and with the 

 anus in front of the posterior extremity on the dorsal side. At no 

 stage in the development has any trace been observed of the tem- 

 porary segmentation which forms so marked a feature in the 

 development of Echiurus. But in at least one member of the 



