INVERTEBEATA, CRYPTOGAMIA, MICROSCOPY, ETC. 881 



endodermal cells surrounded by a ttin mesoderm which, again, is 

 covered by an ectoderm in which, in addition to two cells, supjjlied 

 with long cilia, there is a thickening of several layers of cells ; this is 

 regarded by our author as the first rudiment of the sui^ra-oesophageal 

 ganglion. The embryo now begins to elongate, and from the body, 

 which is divided by the two bands of cilia into three regions, there is 

 developed in the first and semicircular segment an eye on either side, 

 while the whole creature takes on the form of a rhabdocoelous Turbel- 

 larian, and is seen to be possessed of a great power of contractility. 

 The next succeeding stages appear to be somewhat difficult to follow 

 out, and although the mesoderm is seen to divide into an ectodermal 

 and an. endodermal portion, no coelom to speak of is immediately 

 formed. The oil-containing cells of the endolerm give rise to a 

 cavity and the rudiments of the oesophagus are developed. The 

 development of the nervous system goes on at a rapid pace, but the 

 ventral cord does not extend to the hinder end of the body, and the 

 author was not able to observe in it the central canal reported by 

 Greef. Great changes go on in the mesoderm ; the splanchnic plate 

 remains thin, but the somatic becomes greatly thickened, and there is, 

 further, difierentiated an external circular and an internal longitudinal 

 layer ; here ends the sexually indiflfereut stage of the larva, a worm- 

 like creature, containing chloroi^hyll, ciliated over its whole surface 

 and divided by two bands of cilia into three regions, of which the fore- 

 most is supplied with two large pigment spots. The enteron is a 

 closed oil-containing sac, in which the rudiments of the oesophagus 

 may be made out ; the mesoderm is, in the anterior region, formed of 

 a compact mass of vesicular cells, and, in the rest of the body, consists 

 of a thin splanchnic, and of a differentiated somatic plate, in which, 

 besides muscles, there is a mesh of tissue. 



The Female of BonelUa. — The first stages in further differentiation 

 affect the mesh of tissue just mentioned, and the indifferent cells con- 

 tained in it ; the changes which occur convert the " parenchymatous " 

 larva into a " bladder-shaped worm," with a spacious coelom, con- 

 taining fluid. Whence comes this fluid ? the author is not quite able 

 to say, though he offers what he thinks is a plausible explanation. 

 About the time when the conversion of the mesodermal cells is com- 

 pleted there may be seen at the hinder portion of the enteron two 

 closed vesicles given off from the enteron, one on either side, which 

 form, by the breaking through of their walls, a means of communication 

 between the interior of the body and the sea-water ; and the fluid is, 

 as the author supposes, nothing more than sea-water. This change in 

 the characters of the mesoderm does not affect the cephalic portion of 

 it where there is, by a metamorphosis of the portion of the enteron 

 anterior to the oesophagus, developed the cephalic lobe (or so-called 

 " proboscis "). Passing over various points we come to the changes 

 which take place in the epidermis; the ciliated bands, first the 

 posterior and then the anterior, disappear, the cells of the epidermis 

 become flatter, and dermal glands begin to be formed. The muscular 

 bands become so arranged that the circular ones are peripheral, the 

 oblique internal, and the longitudinal median in position. On the 



VOL. II. 3 N 



