330 MisceUaneous. 



degrees extends nearly over one third of the surface of the ovum ; 

 it closes rapidly, thus englobing the exodermic elements in the 

 ventral part of the embryo. The prostoma is still visible, after the 

 disappearance of the furrow, at the inferior extremity of tlie embryo, 

 in the neighbourhood of the point where the definitive anus will 

 subsequently be formed. From this moment the egg becomes elon- 

 gated in the direction of an axis passing through the centre and the 

 prostoma. The cavity of segmentation is more and more visible 

 between the transparent exoderm and the deep red entoderm. 



The embryo then assumes the form trochospha;ra. On each side 

 of the anterior part two cells of the exoderm give origin to crystal- 

 lines, which are soon surrounded at their base by a red pigment. 

 Towards the anterior third there is produced round the body an 

 invagination of the cylindrical cells of the exoderm. The invagi- 

 nated cells become more rcfringent and contractile ; then the inva- 

 gination returning, they reappear furnished with long llagella. It 

 is at this point that the embryo issues from the e^^ ; but whilst in 

 some Annelides I^PhyUodoce for example) the trochotipjiuera swims 

 freely in the water, in Sahnacina the embryo at this stage remains 

 stiU under the maternal fold ; and it is onlj by breaking the cormi 

 that we can follow these first phases of the development. The 

 embryo is slightly bent upon itself; the convex (dorsal) part con- 

 tains the nutritive elements ; the mouth forms on the ventral 

 surface, a little below the vibratile cincture. The part of the 

 embryo above the cincture becomes differentiated into a rounded 

 head no longer containing endodermic elements. 



The larva at the moment of its quitting the maternal tube to 

 swim freely possesses the following parts : — 1, a rounded head, 

 containing the four eyes, and furnished at the anterior part with 

 three rigid cilia ; 2, a cervical part, narrower than the head, having 

 at the cincture long llagella, below which are other, smaller and 

 more numerous cilia, and on the ventral surface the mouth, the 

 circular aperture of which is likewise margined with vibratile cilia ; 

 3, the mantle, formed by a fold of the exoderm, which descends like 

 an apron over the ventral part and rises on the dorsal surface into 

 two epaidet-like organs: the head and neck may conceal themselves 

 in part beneath this exodermic fold ; 4, beneath the mantle, and in 

 part concealed by it, at least on the ventral side, there is a portion 

 of the body as wide as the head, which I shall call the thoracic 

 portion^ because it represents the thorax of the adult animal, or 

 rather the first three segments of the thorax. This part bears three 

 pairs of bundles of setae. Each bundle contains two setae ; and the 

 setae of the first bundles are dissimilar. At the base of the second 

 and third pairs of bundles we observe glands (two to each bundle) 

 with granular contents, belonging to the exoderm ; below the second 

 pair there are four hooks (unciform plates); below the third pair 

 three hooks. At the extremity of the body of the larva there is 

 also on each side a strong hook, and in the vicinity of the anus two 

 long, rigid cilia. The whole anterior ventral part of the body of the 

 embryo contains large cells ^\■ith a very distinct and refringeut 



