490 



ZOOLOGY 



SECT. 



group the mesoderm undergoes segmentation, and the coelome is 

 formed by the coalescence of cavities which appear in these 

 segments. 



Distribution, Affinities, etc. The Sipunculoidea are all 

 marine. They are only capable of very slow creeping locomotion, 

 and live for the most part either in natural rock-fissures, or in 

 interstices between masses of fixed animals such as mussels or 

 ascidians, or in discarded shells of Mollusca or tubes of Polychseta, 

 or in burrows which they excavate for themselves either in sand or 

 mud, coral or rock. Their distribution is general ; and they occur 

 at considerable depths as well as in shallow water. 



The resemblances between the Sipunculoidea and the Echiurida 

 have been and are regarded by many authors as sufficiently impor- 

 tant to justify the inclusion of both in one 

 class to which the name Gephyrea was given. 

 But apart from the embryological history, 

 the introvert of the Sipunculoidea with its 

 crown of tentacles, the forward dorsal position 

 of the anus, and the absence of true setae 

 are all points in which the members of that 

 class diverge from the Echiurida and the 

 Chsetopoda. At the same time it is in the 

 Echiurida probably that the Sipunculoidea 

 find their nearest known relatives. 



The Priapuloidea, often classed with the Sipun- 

 culoidea, may be mentioned here. Priapulus (Fig. 

 405), species of which have been found widely 

 distributed at moderate depths in extra -tropical 

 seas, is a cylindrical worm, unsegmented but with 

 numerous superficial annulations in the posterior 

 half. At the anterior end is the mouth and at the 

 posterior the anus. There is a short introvert, but 

 no oral tentacles or tentacular fold. At the posterior 

 end is a hollow single or double process beset with 

 hollow papillae (resp.) : the lumen of this is in com- 

 munication with the ccelome, and the whole appa- 

 ratus is doubtless a respiratory organ or branchia. 

 The alimentary canal, which is not coiled, consists 

 of a muscular pharynx, a thin-walled intestine, 

 and a rounded rectum. There is no blood- vascular 

 system. The nervous system is in close connection 

 with the epidermis : it consists of a simple ventral 

 nerve-cord and a circum-cesophageal ring without 



FIG. 405.-Pria P ulus, entire brain-enlargement. The sexes are distinct The 



animal, resp. posterior papillae, ovaries or testes are a pair of masses of follicles 



(After Ehlers.) each having its main duct which opens on the 



exterior close to the anus. The genital ducts with 



their branches seem to act as excretory organs. The development is 



unknown. 



e&b 



