INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 565 



doubt tlie larva of Pohjgordius ; of the six stages into whicli the author 

 divides the history of this form the first, or that prior to segmentation, 

 is the most interesting. Small anJ transparent, the larva is more broad 

 than long, and is divided into two parts by two parallel circlets of cilia 

 in the middle line ; the mouth is placed between these and the anus, 

 which is found at the inferior pole of the body ; superiorly there are 

 two eye-spots ; the cuticle is, for the most part thin, and in the region 

 of the trunk forms a layer of cubical cells ; the region of the anterior 

 circlet is invested by a thick cuticle, which is traversed by pore-canals 

 for the long cilia, which seem to be nourished from a mass of cells set 

 anteriorly, and containing fatty and albuminous granules : the cuticle 

 of the cells of the posterior circlet is but slightly thickened, but the 

 same pore-canals are again to be observed The pigment spots are 

 dark brown, and the region with which they are connected will form 

 in time the supra-oesophageal ganglia ; connected with this, on each 

 side, are three nerves, one of which is much larger than the others, and 

 all of which go to form the peripheral nervous system. As in Crio- 

 drilus, the primitive segments are first developed in the mesodermal 

 stripes, and as, not only in it, but in Unio among Lamellibranchs, and in 

 PediceUina among the Bryozoa, there are, for a long time, two distinct 

 primitive mesodermal cells at the posterior end of these stripes. A 

 number of muscular fibres are early developed in the head, as is also a 

 delicate ciliated canal on either side, which forms the head-kidney, 

 and gives rise in the future to the " segmental organs " of the trunk. 

 This canal communicates with the body-cavity by the funnel-shaped 

 orifice so characteristic of this organ. 



As to the enteric canal, we have to observe that, as so often happens, 

 it is the mid-gut only that is developed from the endoderm ; there is a 

 diaphragmatic elevation in the lumen of the fore-gut which is provided 

 on the side most remote from the mouth with long cilia, which appear 

 to be able to act as a kind of sieve ; the hind-gut has muscular fibres 

 connected with its walls, which are apparently of assistance in the 

 special function of this part. 



The succeeding stages are characterized (2) by the appearance of 

 the primitive segments of the trunk, the further development of the 

 mesoderm, and of the posterior circlet of cilia ; (3) the greater elonga- 

 tion of the larva and the differentiation of the ventral cord and 

 antennae, as well as of a ciliated pit on either side of the head, which 

 seems to represent the future olfactory organ. (4) In this stage the 

 walls of the enteric canal become strengthened by the apposition of 

 the proper division of the mesoderm, and the posterior circlet of cilia 

 becomes greatly developed. (5) The cuticle of the trunk becomes 

 thicker, and the region of the head becomes much more like what it 

 will be in the adult, while its circlets of cilia begin to disappear. 

 (6) Two distinct regions are apparent in the head, the tentacles 

 project on either side, the walls of the mid-gut are considerably 

 thickened, and there is still a mass of rounded indifferent cells, which 

 appear to be the rudiments of the generative products. 



As to the systematic position of Polygordius, Dr. Hatschek agrees 

 with Uljanin in regarding it as belonging to the Annelides, although 



