52 ORIGIN OF MESOBLAST. 



3. Triploblastic Larvae. 



There is great variety in the external form of the triplo- 

 blastic larvae and a description of each will be found in the 

 account of the phyla in which they occur. The most important 

 are: — Bipinnaria and Pliiteus {Ec/iinodermata), Tornaria 

 [Balanoglossus)^ Trochophore ( Annelida ^ud MoUusca)^ Naup- 

 lius {Crustacea)^ ChordiUa {Atriozod)^ Tadpole {Ajnphibia), 



The third layer or mesoblast develops from the hypoblast 

 in the same variety of manner as does the hypoblast from 

 the archiblast. Hence the mesoblast may arise by invag- 

 ination, ingression, or delamination. 



With the origin of the mesoblast the diploblastic larva 

 becomes piano-symmetric; hence the mesoblast usually shows 

 a more or less paired arrangement. The hypoblast arises 

 by one invagination or by one ingrowth, but the mesoblast 

 arises by never less than two rudiments, which soon become 

 arranged laterally. 



There is great variety in the details, but after the 

 mesoblast is established it usually shows the following 

 characters : — It consists of a more or less complex double 

 layer of cells, of which the outer layer lines the epiblast and 

 the inner covers the hypoblast. These two layers enclose a 

 spacious cavity called the coelom, which usually is filled with 

 a nutrient fluid. The coelom is not usually continuous but 

 it may be divided in the median plane by dorsal and 

 ventral mesenteries, which are double, and serve to support 

 the hypoblastic canal ; or it may be divided up by lateral 

 mesenteries or septa running transversely to the long axis of 

 the organism. The mesoblastic walls later form the muscles, 

 skeletal tissue, gonads, and partly the excretory organs; 

 and the coelom often communicates with the exterior by 

 paired canals called nephridia.* 



The cxlom is therefore a cavity entirely surrounded by 

 mesoblast ; its walls give rise to the muscular, skeletal and 

 reproductive systems; and it usually communicates by 

 paired apertures or canals to the exterior. 



It may arise in continuity with the cavity of the hypoblast or 

 archenteron which is obviously the case when the mesoblast 

 arises by invagination. This origin is called enteroccclic, 



* This should be compared with the types of structure in Chap. III. 



