THE DEVELOPMENT OF APLYSIA PUNCTATA. 527 



is typical in both Annelids and Molluscs. Whenever in these 

 groups it does not occui', it is easy to account for its dis- 

 appearance; in Nassa, for example, and in the Cephalopoda 

 the form of cleavage is clearly correlated with the large 

 amount of yolk present in the egg. lliere is often a strikingly 

 close resemblance between the cell lineages that have been 

 worked out in the two phyla, but too great stress should not 

 be laid on this point, since the resemblances between the 

 cleavage patterns may be taken to indicate a similarity of 

 physical and mechanical conditions in the egg, rather than of 

 any close phylogenetic relationship. However that may be, 

 Carazzi's work on the cell lineage of Aplysia only adds 

 another to the already long list of remarkable parallels in 

 this respect between the two groups. 



Our own work is more directly concerned with the origin 

 and fate of the mesoderm in Annelids and Molluscs. In all 

 Annelids in which the cell lineage has been investigated, the 

 cell known as 4d gives rise to the most important part of the 

 mesoderm, and sometimes to all the mesoderm, as in Aplysia. 

 In some other forms the so-called larval mesoderm derived 

 from the ectoderm contributes to a greater or less extent to the 

 structure of the larva. In Annelids the subsequent history 

 of the mesoderm is well known ; and it is interesting to find 

 that in many Molluscs well-developed mesodermal bands are 

 found, in all ways comparable to those in Annelids. As 

 might be expected, the best examples of mesodermal bands 

 occur in the more primitive groups of Molluscs. Kowalevsky 

 has described them in Chiton polii, and Heath in Ischno- 

 chiton. Among the Solenogastres also we Hndin Dondersia 

 and Proneomenia that the bands are unusually distinct, and 

 Patten figured them clearly in Patella. But in the more 

 modified groups of the Mollusca one could hardly speak of 

 mesoderm bands except on the analogy of the less specialised 

 forms. This is the case in Aplysia, where the bands are 

 never clearly defined and soon break up into scattered cells. 



Turning to the development of the coelom, it is here that 

 we find the first essential difference between Annelids and 



