DEVELOPMENT OF CLEPSINE. 221 



have sunk slightly inwards. The small cells are ectodermic, they con- 

 tribute to the formation of the epidermis, and apparently form also the 

 ectoderm of the head region. The ectodermic structures of the body, on 

 the other hand, are formed by the eight neuro-nephroblasts. These, 

 together with the two mesoblasts underlying them, undergo continuous 

 division in a forward direction, and so produce long rows of cells — the 

 germ bands. The two germ bands are widely separated posteriorly, 

 but commence to unite anteriorly, the union travelling liackward. As 

 the neuro-nephroblasts must lie regarded as ectodermic in origin, we 

 see that the spreading of the micromeres over the surface of the egg, and 

 the union of the germ bands, constitute together the delayed epibolic 

 gastrulation. 



Each germ band consists of three layers, first a thin epidermic stratum, 

 then the layer of the neuro-nephroblasts, and finally the mesodermic 

 layer. Of the neuro-nephroblasts, the inner two form the ventral nerve 

 chain, the next two on either side the nephridia, while the fate of the 

 outer on each side is unknown. The mesoblast rows give rise to the 

 mesoderm, the gut is formed by the entoblasts, and an anterior ectodermic 

 invagination forms the pharynx. At this stage the embryos leave the 

 cocoon and attach themselves to the ventral surface of the mother. 

 A little later the form of the body becomes approximated to that 

 of the adult, and an anus is formed by the fusion of ectoderm and 

 endoderm. 



The most interesting point about this development is that, although 

 the method of gastrulation differs widely from that of Luinbykus, the 

 history of the germ bands shows very marked resemblances in the two 

 forms. It can hardly be that these resemblances are due to adaptation, 

 so that we must consider that development confirms the view which is 

 otherwise probable, that the leeches are true Annelida. In Clcpsine the 

 eight neuro-nephroblasts are not, as in Litmbi-icits^ obviously ectodermic 

 in origin, but are early covered over by the ectodermic micromeres. 

 Both analogy and the future course of development, however, prove that 

 they do, nevertheless, belong to the outer layer, and that their position is 

 due to a hastening of events. 



Classification. 



t. Rhychobdellidfe, in which the fore part of the pharynx can be 



protruded as a proboscis. There is an anterior as well as 

 a posterior sucker. The blood plasma is colourless. The ova 



are large and rich in yolk ; the embryos are hatched at an 



advanced stage, and soon leave the cocoon, which contains no 

 albuminous fluid. 



e.g., Clepsiiie, Poniobdella, Branchellion. 



2. Gnathobdellidre, in which there is no proboscis, but the pharynx 

 usually bears three tooth plates. The mouth is suctorial. The 

 blood plasma is red. The ova are small and without much yolk ; 

 the embryos are hatched at an early stage, and swim about in the 

 nutritive albuminous fluid of the cocoon. 



e.g., Hiriido, fficnwpis, Hicmadipsa, Anlastoiiia, Nephdis. 



