ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
37 
yolk witli which it leaves the egg, and which is only gradually absorbed, 
the young animal does not take food from outside till very late, and the 
maxillary palps are, conseq[uently, for a long time without setas. The 
freshwater form is to be regarded as having been derived from the 
marine ; and its peculiarities are analogous to those which we often find 
in other freshwater animals which have allies in the sea. The pecu- 
liarity of the case lies in the fact that the adults of the two forms have 
remained almost exactly alike, while the development has become so 
very different. 
Vermes, 
a, Annelida. 
Development of Annelids.^ — M, L. Eoule has a lengthy memoir on 
the development of Annelida, based chiefly on a study of Enchytrseoides 
marioni. The embryos pass through the early stages of their develop- 
ment in cocoons, and escaj^e when they have from fourteen to fifteen 
rings. Segmentation is complete and somewhat unequal ; as a rule 
there is no cavity comparable to a blastocoel ; a planula is formed by 
tangential division of the ceils, and the interior is at first a mesoendo- 
blast, and is extensive. As an archenteron appears the endoblast 
becomes distinct as a single layer of cells from the five or six rows that 
form the mesoblast. We have already noted | the author’s description 
of the formation of the coelom. As the embryo elongates and becomes 
cylindrical the archenteron and coelom increase in size ; the ectoblast of 
the anterior end of the body thickens to form the cephalic plate, and a 
similar medullary plate appears on the ventral surface. As the embryo 
elongates it becomes narrower, and four or five rings apj)ear in the 
anterior region of the body ; septa begin to extend from tJie somato- 
pleure to the splanchnopleure, and the cephalic lobe becomes marked 
off as the most anterior segment of the body. The ectoblast becomes 
depressed in relation to the septa, so that an external annulation corre- 
sponds to the internal segmentation of the mesoblast and coelom. The 
dorsal and ventral portion of the central nervous system next become 
united. Behind the last ring there is a large mass of mesoblast in 
which closed cavities are hollowed out by pairs, on either side of the 
digestive tract. The setm are formed at the expense of the ectoblast, 
and the muscles which move them from the somatopleure. The external 
and sub-ectoblastic cells of the somatopleure begin to be differentiated 
into smooth muscular fibres by the formation of contractile substance 
at their periphery ; they thus become fibre-cells, comparable to those of 
Molluscs. 
The nephridia appear in the form of a continuous cord, which 
becomes differentiated in the deep region of the soinatoj)leure ; this cord 
divides into groups of four or five cells, which become connected with 
the sejDta. The cells of the group fuse, and vibratile canals become 
hollowed out in the mass. After expulsion from the cocoon the body of 
the embryos continues to elongate, and two blood-vessels begin to 
aj)pear ; these trunks join and fuse ; at first each trunk is represented 
* Ann. Sci. Nat., vii. (1889) pp. 107-412 (15 pis.). 
t See this Journal, 1889, p. 887. 
