52 ORIGIN OF MESOBLAST. 
3. Triploblastic Larve. 
_There is great variety in the external form of the triplo- 
blastic larvee and a description of each will be found in the 
account of the phyla in which they occur. The most important 
are:— Bipinnaria and Pluteus (Echinodermata), Tornaria 
(Balanoglossus), Trochophore (Annelida and Mollusca), Naup- 
lius (Crustacea), Chordula (Atriozoa), Tadpole (Amphibia). 
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 plano-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 mwsually 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 ce/om, which usually is filled with 
a nutrient fluid. The ccelom 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 ccelom often communicates with the exterior by 
paired canals called nephridia.* 
The calom 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 enxéerocalic. 
* This should be compared with the types of structure in Chap. III. 
