SIPUNCULOIDEA. 



537 



oral vessel, which sends prolongations into the tentacles ; it gives off 

 no branches. There is often present in addition a vessel on the 

 ventral side of the oesophagus, which also opens into the circumoral 

 vessel. Inasmuch as this system gives no branches to the viscera, 

 it cannot be regarded as a nutritive or respiratory organ; it is probably 

 mainly concerned with expanding the tentacles. A vascular system 

 is absent in Onclmesoma and in Tylosoma. There are no special 

 organs of respiration, unless the tentacles are such. 



The nephridia are two in number ; they are often called the broivn 

 tubes, and open externally on the ventral surface in the anterior 

 region. They project backwards in the body-cavity from their attach- 

 ment to the skin, and they possess an internal opening into the 

 perivisceral coelom at their anterior end close to the internal opening. 

 They have glandular walls, and serve to transmit the- reproductive 

 cells to the exterior. 



Mz 



FIG. 434. Stage of Sipunmhts embryos, in which the cephalic and ventral plates are beginning 

 to fuse with one another. The vitelline membrane and the cilia of the serosa which perforate 

 it are omitted (after Hatschek). a, In transverse section ; Kp cephalic, Ep ventral plate ; 

 E endoderm ; Ms mesoderm ; ,S serosa. b, In median longitudinal section ; Oe oesophagus ; 

 Mz pole cells of the mesoderm. 



The sexes are generally separate, and the reproductive glands are 

 simply proliferations of the coelomic epithelium. In Sipunculus 

 these proliferations form an annular thickening of the peritoneal 

 lining at the point of attachment of the retractor muscle to the 

 body-wall. The reproductive cells are dehisced from the generative 

 ridge before -they are ripe, and mature in the body-cavity. 



Fertilization is probably external. In Sipunculus, the development 

 of which was observed by Hatschek on larvae taken in the Pantano, 

 near the lighthouse at the north end of the Straits of Messina, there 

 is an invaginate gastrula. The blastopore closes; the mouth and 

 anus being new formations. There are two primary mesoblast cells 



