MOLLUSCS AND ANNELIDS. 403 



the cephalic or oral end the oldest. 7. The neural surface of the embryo is laid 

 down and differentiated earlier than the haemal surface, the difference between 

 the time of formation and the amount of specialization in the two surfaces being 

 governed largely by the volume and distribution of the yolk mass. See Chapter 

 XIII, p. 219. 



There is a well marked difference between the embryonic processes in the 

 craniates and acraniates on the one side, and the molluscs and the annelids on the 

 other, due to a prevailing difference in the volume of yolk in the four groups, the 

 period at which the embryo is liberated, and in the unequal emphasis placed on 

 growth at the cephalic and caudal ends of the body. 



The principal features of these groups may be summarized as follows: 



A. Molluscs and Annelids. — In these animals we have a true gastrulation 

 in its original meaning, for the blastular infolding gives rise to endoderm only, 

 and the blastopore, without noticeable elongation persists as the mouth. The 

 mesoderm arises from cells lodged in or near the posterior lip of the blastopore. 

 The young usually escape from the egg as so-called trochospheres, a larval form 

 representing the coelenterate phase of their development. Its characteristic 

 feature is a transverse, or equatorial ciliated band encircling the principal axes 

 between the mouth and the apical plate. (Fig. 267.) 



In the annelids the trochosphere forms only the head, the body always 

 arising as a new local outgrowth, not by the elongation of the trochosphere 

 as a whole. When there is a considerable amount of yolk present, the trocho- 

 sphere stage may be passed within the egg membranes; in these cases the trunk is 

 formed by the rapid proliferation of a special group of large terminal cells, or telo- 

 blasts, of which there may be several kinds, each giving rise to a linear series of 

 some particular kind of organ, as nerve cords, nephridia, entoderm, etc. 



In the molluscs the trochosphere is transformed into the adult with little or no 

 axial elongation. 



The molluscs and annelids, therefore, are characterized by small or medium 

 sized eggs that pass through a true gastrulation; the blastopore persists as the 

 mouth, and the larva is a coelenterate-like trochosphere. The germ layers of the 

 trunk arise simultaneously with the progress of apical growth, no one layer arises 

 from another, the mesoderm never forms a part of the functional enteron, and 

 the mesocoele never opens into the enterocoele. 



B. Craniates. — In the arachnid-vertebrate stock, the embryo grows film- 

 like over the surface of a large body of yolk. It passes rapidly through the gas- 

 trula and trochosphere stages, and is retained within the egg membrane till the 

 trunk is well developed, rarely being liberated with less than fifteen or twenty 

 highly specialized metameres. (Figs. 25 to 32.) 



The only recognizable remnants of the gastrula are found in the primitive 

 cumulus, or circular germ disc, at the point where the primitive mouth and pro- 

 cephalic lobes are formed. (Figs. 25, 269.) The body, or trunk, is a new formation 

 that has no real homologue in a coelenterate, and it is formed solely by the multi- 



