INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 967 



sots of vessels. Then there are the tentacles, supj)liecl by the tentacular 

 vessels, and lastly, there are the generative organs which follow tho 

 course of the ctenophoral vessels, in the walls of which the products 

 are developed. 



There are well-marked differences between the three layers of the 

 body. The ectoderm is the most widely distributed, inasmuch as it 

 does not only invest the body, but also the stomach. It is, further, 

 the layer which undergoes the greatest amount of differentiation, 

 formiug not only investing, but also glandular, pigment, ciliated and 

 sensory cells, in addition to nerves and muscles. The most important 

 organs are nothing but sjiecially differentiated parts of the ectoderm. 



The sensory cells are of two kinds, which so far agree in structure 

 that they are always provided with stiff' processes, which have evidently 

 a tactile function. The most ordinary form of sensory cells is that 

 which carries a number of small tactile processes. Of these there may 

 be (Eucharis multicornis) as many as seven ; others bear only one pro- 

 cess, and that of considerable length and thickness. The author is 

 confident as to the presence in the epidermis of a nervous layer. Other 

 ectodermal structures must be here passed over, although they exhibit 

 many j^oints of considerable interest. 



The mesoderm forms the great mass of the body. It is gelatinous, 

 and in Callianira, EucJiaris, and Cydippe is very soft, while in the 

 Cestidae and Beroidse it is much firmer. It does not develop any sup- 

 porting lamella, and although in itself completely structureless, it 

 contains a number of variously differentiated cells. In Beroe ovatus 

 there are found to be either muscular fibres, nerve-fibres, or connective- 

 tissiic corpuscles. Although these are, when most pronounced, easy 

 enough to distinguish from one another, there are also others which 

 seem to be intermediate in character. The muscles are either radial, 

 circular, or longitudinal ; each fibre consists of an axial and a cortical 

 substance, together with a sarcolcmma. 



The endoderm is comparatively uniform in character. It would 

 appear to consist of a single layer of ci)ithelial cells, flattened on ono 

 side ; the epithelium of the vessels is richly ciliated ; distinct and 

 well-marked stomata are to be observed, by means of which fluid can 

 jKiss to the mesoderm, without any special modification of the endo- 

 dcrmal cells being necessary. They are boimdod by a rosette of cells 

 which call to mind, by tlicir arrangement, the structure of ciliated 

 infundibula. No nerves or muscles were to be observed in this layer. 



Medusae and Hydroid Polyps living in Fresh Water. * — 

 Professor Lankoster jxjints out that the tolerance by marine animals of 

 fresh water is a much more freipicntly observed fact in all classes, 

 than the tolcriince of seik water by lacustrine or fluviatile forms. It is 

 undeniable that existing fiesh-water forms have been developed by 

 adaptation from marine forms, whilst it is difficult to cite any instance 

 in which adaptation in the opposite direction ajipears to have taken 

 place, some few marine 01igocha;tous ChiDtopods and Pulmouato 

 Gasteropods being i)erhaps such instances. 



* 'Qiuirt. Jouru. Mkt. yd.,' xx. (18S0) pp. 483-5. 



