282 BLAHTOPORE. 



a metamorphosis. Rot if era*. Mollusca Cephalopoda. Polyzoa*. Brachio- 

 poda Argiope, Terebratula, Terebratulina. Cluftopoda Euaxes. Discophora 

 Clepsine. Gephyrea Bouellia*. Cluctoynatha. Crustacea Decapoda. 



Chordata. 



The forms which have been classed togetber under tbe last bead- 

 ing vary consider-ably in tbe character of the blastopore. In some 

 cases the fact of its not coinciding either with the mouth or anus 

 appears to be due simply to the presence of a large amount of food- 

 yolk. The cases of the Cephalopoda, of Euaxes, and perhaps of 

 Clepsine and Bonellia, are to be explained in this way : in the case of 

 all these forms, except Bonellia, the blastopore has the form of an 

 elongated slit along the ventral surface. This type of blastopore 

 is characteristic of the Mollusca generally, of the Polyzoa, of the Ne- 

 matelmintb.es, and very possibly of the Chcetopoda and Discophora. 

 In the Crmetognatha (fig. 209 B) the blastopore is situated, so far as 

 can be determined, behind the future anus. In many Decapoda the 

 blastopore is placed behind, but not far from, the anus. In the 

 Chordata it is also placed posteriorly to the anus, and, remarkably 

 enough, remains, iu a large number of forms, for some time in con- 

 nection with the neural tube by a neurenteric canal. 



The great variations in the character of the gastrula, indicated in 

 the above summary, go far to shew that if the gastruloe, as we find 

 them in most types, have any ancestral characters, these characters can 

 only be of the most general kind. This may best be shewn by the 

 consideration of a few striking instances. The blastopore in Mol- 

 lusca has an elongated slit-like form, extending along the ventral 

 surface from the mouth to the anus. In Echinodermata it is a narrow 

 pore, remaining as the anus. In most Chaetopoda it is a pore remain- 

 ing as the mouth, but in some as the anus. In Chordata it is a pos- 

 teriorly-placed pore, opening into both the archenteron and the neural 

 canal. 



It is clearly out of the question to explain all these differences as 

 having connection with the characters of ancestral forms. Many of 

 them can only be accounted for as secondary adaptations for the 

 convenience of development. 



The epibolic gastrula of Mammalia (vide pp. 178 and 242) is a still 

 more striking case of a secondary embryonic process, and is not directly 

 derived from the gastrula of the lower Chordata. It probably origi- 

 nated in connection with the loss of food-yolk which took place on 

 the establishment of a placental nutrition for the foetus. The epibo- 

 lic gastrula of the Scorpion, of Isopocls, and of other Arthropoda, 

 seems also to be a derived gastrula. These instances of secondary 

 gastrula3 are very probably by no means isolated, and should serve as 

 a warning against laying too much stress upon the frequency of the 

 occurrence of invagination. The great influence of the food-volk 

 upon the early development might be illustrated by numerous exam- 

 ples, especially amongst the Chordata (vide Chapter XL). 



If the descendants of a form with a large amount of food-yolk in 



