224 SIDNEY F. HARMER. 



sucker ou the one hand, and from the epithelium lining the 

 mantle-cavity on the other. 



I formerly assumed (15, p. 455), on the authority of Ostrou- 

 moff's statements (25, pi. vi, fig. 1), that the cavity lined by 

 the inner cells represented the alimentary canal of the larva. 

 But, taking into account the manner in which the larvae are 

 developed, it appears to me doubtful whether any representa- 

 tive of the endoderm occurs in them. It appears to me to be 

 satisfactorily established that a young polypide-bud in any 

 Polyzoon is developed at the expense of two layers, viz. the 

 ectoderm and a layer of funicular tissue which may be re- 

 garded as mesoderm.^ The metamorphosis of the larva of 

 Cyclostomata has been described by Barrois (3) and by Ostrou- 

 moff (25). The observations of Barrois show that the pro- 

 cesses of fixation and of metamorphosis take place essentially 

 as in other Gymnolsemata. The larva fixes by the eversion of 

 its sucker, its mantle being rolled downwards so as to come into 

 contact with the flattened plate formed by the eversion of the 

 sucker, and the greater part of the larval tissues undergo a 

 process of histolysis. The larva thus enters into the condition 

 of a zooecium containing a " brown body," and the young poly- 

 pide is produced by an invagination of the body-wall from the 

 centre of the surface opposite to the basal surface. While the 

 inner layer of the bud is formed by an invagination of the 

 ectoderm, Barrois was unfortunately unable to trace the history 

 of its outer layer. 



Ostroumoff is but little more definite on this point. The 

 inner layer of the bud is formed, according to this observer, 

 not as an invagination, but as a plate of cells split off from the 

 aboral ectoderm. The edges of this plate curve round, so as 

 to transform the plate into a sac, to the outer side of which 

 '' mesenchym-cells " apply themselves, and form the outer 

 layer of the bud. The origin of these '' mesenchym-cells " is 

 not traced. It is recognised that the ''alimentary canal" of 

 the earlier stage disappears, but there is nothing to show how 

 its cells are related to the "mesenchym-cells'^ shown in 

 ' Cf. especially Sceliger, Nos. 32 and 33. 



