624 THE ANIMAL KINGDOM. 



of its development are the following. There is a large prostomium, which becomes 

 the ' proboscis ' of the adult. The mouth is ventral, the anus posterior and terminal. 

 There is a double praeoral ring of cilia, a postoral ring, and between the two a 

 ciliated adoral zone, which is continued as a ciliated ventral furrow from mouth to 

 anus. There is also praeanal ciliated ring. The mesoderm is segmented. Fifteen 

 somites with a terminal piece are indicated by septa internally, by ciliated rings 

 externally. These rings are replaced in the young Echiurus by zones of spinous 

 tubercules, and in somites fourteen and fifteen by the circles of setae. Four 

 zones of tubercules are formed anteriorly, in addition to the first of the series 

 referred to. The nervous system is developed (i) from an ectodermal thickening 

 at the apex of the prostomium, from which commissures grow down to become 

 connected with (2) the ventral cord. The latter arises at first as segmentally- 

 arranged thickenings of the epiblast, which subsequently fuse. The fibres of the 

 cord are derived from the cells of the ventral furrow, which is perhaps invaginated. 

 There is a pair of provisional cephalic nephridia, ciliated, branched, and the 

 terminations of the branches ending in aborted flame-cells. The anal sacs originate 

 from the mesoblast as two tubes, opening by an internal funnel and an external 

 pore, situated close to the anus. Their communication with the rectum in the adult 

 is therefore secondary. 



Thalassema mellita (Conn) has an invaginate gastrula. A ciliated band 

 appears round the blastopore, and is replaced in the Trochosphere by a praeoral 

 ring or row of long powerful cilia. A postoral and praeanal band are subsequently 

 formed, and a ventral ciliated furrow. A group of stiff cilia (? sense hairs) mark the 

 apex of the prostomium. The alimentary canal is formed entirely from the in- 

 vaginated archenteron. The blastopore persists as the mouth. The mesoderm 

 forms two ventral bands (as in Echiurus and Chaetopoda), but stellate mesodermal 

 cells are also derived from the hypoblast, as in Holothurians, &c. 



The larva of Bonellia viridis possesses two rings of cilia : it also becomes 

 ciliated all over. The mouth probably appears behind the first ring, but is formed 

 after its disappearance. The ventral cord shows no ganglionic rudiments. The 

 anal pouches are said to develope from the rectum (Spengel). 



Sipunculus nudus has an invaginate gastrula. During development the epi- 

 blast forms an amnion, which does not cover the apex of the prostomium, indicated 

 by a tuft of long cilia. There are no traces of somites. A postoral band of cilia 

 is present, and the anterior nephridia (uterine pouches) appear early. The anus 

 and proctodaeum are dorsal and at first nearly posterior, but the real posterior apex 

 of the body is denoted by a group of sense cells with hairs. The ventral cord is an 

 epiblastic thickening. There is a glandular appendage opening at the posterior 

 margin of the mouth, and a pharyngeal apparatus attached to the ventral wall of 

 the oesophagus. Both structures are lost during the growth of the larva. Ciliated 

 organs, like the Topfchen, but not free, appear very early in the parietal coelomic 

 epithelium of the body. During the growth of the larva, the prostomial process 

 becomes shorter and shorter, and the anus more and more dorsal. The tentacles 

 appear on either side of the mouth, and have no connection with the postoral ring 

 of cilia. 



In Phascolosoma elongatum there is an invaginate gastrula. The apex of the 

 prostomium bears sensory cells with hairs. A postoral ring of long cilia appears^ 



