280 
Proceedings of Royal Society of Edinburgh. [sess. 
Ontogenetic. 
1. Those species with larval forms have simple larvae, with 
the body divided up into three segments, one pre-oral and two 
post-oral. 
2. The mesoderm arises by arclienteric invagination, or by a 
simple modification of it. 
3. There are often in the free swimming larvae three ciliated 
bands, one pre-oral and two post-oral. 
In order that species retaining this archimeric segmentation 
should have survived, they must of necessity have adopted certain 
habitats, in which naturalists are wont to find primitive forms. 
In this category are pelagic, deep-sea, burrowing and sedentary 
habitats, and amongst the animals affecting these are found forms 
agreeing with our type. In the case of sedentary animals, the 
degeneration and great anatomical modification involved, often 
disguise, in the adult, the features we wish to find, but the 
ontogeny comes to our assistance. 
Chcetognatlia {Pelagic). 
The isolated position and primitive characters of this group are 
usually acknowledged, and in their anatomy we find the coelome 
divided into paired protocoeles, mostly muscular, paired mesocoeles, 
and paired metacoeles (fig. 1). The mouth is terminal, and the 
protocoeles are greatly reduced. The mesocoeles are predominant, 
and contain the female sexual elements, which may be correlated 
with the post-anal position of the metacoeles, in which the male (or 
katabolic) sexual elements arise. This condition may be a primi- 
tive survival of Stage II., but is more likely a return to this con- 
dition from Stage III. by a movement formed of the alimentary 
canal and anus, correlated with the muscular development of the 
metacoeles as a means of locomotion through the tail-fin, as is 
paralleled in the fishes. 
The nervous system presents the central ganglion in the proto- 
mere, the post-oral ring, and the mesomeric ganglion, all in con- 
tinuity with the ectoderm. 
The ontogeny is usually regarded as direct, but the coelome still 
arises by archenteric diverticula. The protocoeles are derived from 
