488 ZOOLOGY SECT. 



nearly straight tubes, of a brown or yellowish colour, and very 

 mobile in the living condition. Near the external opening, which 

 is situated at the anterior end, is the internal opening into the 

 coelome. The sexes are separate. There are no definite gonads 

 except at a certain season of the year, when cellular elevations 

 developed in the connective - tissue covering the ventral 

 retractor muscles of the introvert represent ovaries or testes 

 as the case may be. These give origin to cells which become 

 detached and develop into the fully-formed sexual elements 

 while floating about in the coelomic fluid. The nephridia act 

 as gonoducts. 



2. DISTINCTIVE CHARACTERS. 



The Sipunculoidea are Annulata with the body devoid of any 

 appearance of segmentation in the adult condition. There is a large 

 coelome, which is not divided into chambers by mesenteries or 

 septa. A blood- vascular system is sometimes present, sometimes 

 absent. The ventral nerve-cord is not composed of a chain of 

 ganglia. There is usually only one pair of nephridia. The sexes 

 are separate, the gonads simple, and the nephridia act as 

 gonoducts. 



The larva is in most cases a trochophore. 



3. GENERAL ORGANISATION. 



The Sipunculoidea are a class of worms whose position among the 

 Annulata is determined more from a consideration of their develop- 

 ment than of their structure in the adult condition, though the 

 latter suggests a tolerably close affinity with the Chaetopoda. The 

 body of a Gephyrean is unsegmented, usually more or less com- 

 pletely cylindrical, broadest behind and narrowing towards the 

 anterior end. The surface is covered with a chitinous cuticle 

 developed often into papillae, or tubercles, or hooks. The anterior 

 part of the body is capable of being invaginated within the part 

 behind ; at the extreme anterior end of this invaginable part or 

 introvert, when it is evaginated, is the mouth, surrounded by a 

 circlet of sometimes pinnate, sometimes simple, tentacles, or by a 

 lobed and plaited tentacular fold. The prostomium is quite 

 rudimentary. The anus lies far forwards on the dorsal surface. 



Body-wall. Beneath the cuticle is an epidermis, which is 

 composed of a single layer of cells. Among the cells are unicellular 

 (rarely multicellular) glands, and sensory cells. The muscular 

 wall of the body consists of external circular and internal longitu- 

 dinal layers, sometimes with oblique and internal circular layers 

 superadded. There is an extensive undivided coelome, lined, as 

 in the case of the Chsetopoda, with a coelomic epithelium, which is 

 sometimes ciliated. Apertures (stomata) lead from the coelome 

 into variously arranged systems of canals in the wall of the body. 



