376 INVERTEBEATA chap. 



where the stomodaeum is eventually formed. It is a triangular 

 plate of cells with the apex directed forwards and is formed of 

 descendants of 2d. Trunk-blastema and head-blastema are con- 

 nected in the mid-dorsal line by a narrow cord of cells — the dorsal 

 cord {d.c. Fig. 304). 



The mother cells of the mesoderm, by this time, have each given 

 rise to an anterior band of four cells and to a minute posterior cell, 

 which lies against the endoderm, and which, as in Mollusca, probably 

 gives rise to the intestine. The closure of the blastopore is effected 

 partly by the forward growth of the trunk-blastema and partly by 

 the in-sinking of the descendants of 2a, 2b, and 2c which give 

 rise to the stomodaeum. 



The embryo becomes a larva by beginning to swim. This 

 happens about twenty-four hours after fertilization of the egg. The 

 larva does not burst the egg-membrane, but carries it about with it. 



The prototroch, composed of the primary and secondary trocho- 

 blasts, is a broad belt of cells covered with minute cilia. There is a 

 well-marked metatroch consisting of a girdle of long cilia. Between 

 prototroch and metatroch is situated the opening of the stomodaeum, 

 which is surrounded by a special girdle of small cilia. In front of 

 the apical plate, to the right and left of the middle line, eye-spots 

 are found {oc, Fig. 305) ; these are situated just above the region 

 where the cells are being invaginated to form the head-blastema. 



Until thirty-six hours after fertilization have elapsed, the larva 

 remains spherical. After that time the posterior portion of the 

 body elongates more rapidly than the anterior portion, and at the 

 beginning of the third day, in the case, at any rate, of Phascolosoma 

 vidgare, it sinks to the bottom. At this period the egg-membrane is 

 at last shed, and underneath it a fine cuticle is now to be seen, which 

 has been mistaken for the persisting egg-membrane but is in reality 

 quite distinct from it. 



Before, however, this happens, very considerable changes occur in 

 the Trochophore larva. The appearance of the apical plate changes, 

 since the sense-organ appears to move to its anterior edge ; this is 

 due to the anterior part of the plate becoming invaginated to form the 

 cerebral ganglion. Eound the edge of the apical plate is found the 

 prae-oral band of cilia ; this has nothing to do with the prototroch, 

 the cells composing which carry quite minute cilia. Behind the proto- 

 troch there is a narrow band of ectoderm from which the mesecto- 

 derm is formed; that is to say, from this band cells are budded inwards 

 into the blastocoele. Some of these cells are transformed into 

 longitudinal accessory retractors (ret.acc, Fig. 306), others become 

 changed into circular muscles. The principal retractors are formed 

 from the cells of the apical plate which bear the prae-oral circle of 

 cilia. These retractors retain throughout life their insertion into the 

 ectoderm at the point where they originated from the apical plate 

 {ret.d, ret.v, Fig. 306). 



Before the Trochophore larva sinks to the bottom the rudiment 



