76 INVERTEBRATE ANIMALS. 



may be taken as a good example, the general structure is briefly 

 as follows : The hydrosoma is perfectly free and is oceanic, 

 being found swimming near the surface in the open ocean. 

 The body is composed of a thick, transparent, gelatinous disk 

 or swimming-bell (the nectocalyx), by the pulsations of which 

 the animal is driven through the water. From the under 

 surface or roof of this bell-shaped disk is suspended a single 

 polypi te (the manubrium), which bears to the disk the same 

 relative position as the clapper does to an ordinary hand- 

 bell. The distal end of the central polypite is furnished with 

 a mouth, the lips of which are often prolonged into four 

 longer or shorter lobes or processes. The mouth opens into a 

 digestive sac, occupying the axis of the polypite ; and from 

 the upper end of this proceed four radiating canals, which 

 run in the substance of the disk to its margin, where they are 

 united by a single circular vessel, the whole system con- 

 stituting the so-called " gastro-vascular " canals. The margin 

 of the bell is narrowed by a kind of shelf, which runs round 

 the whole circumference, leaving a central aperture, and which 

 is known as the " veil." From the margin of the disk hang 

 more or less numerous tentacles, which are hollow processes 

 of the ectoderm and endoderm, and which communicate with 

 the circular vessel of the canal-system. Also round the cir- 

 cumference of the swimming-bell are disposed certain " margi- 

 nal bodies," which are doubtless organs of sense. Some of 

 these marginal bodies consist of little rounded sacs or " vesi- 

 cles," filled with a transparent fluid, and containing mineral 

 particles, apparently of carbonate of lime. These are probably 

 rudimentary organs of hearing. Others of the marginal bodies 

 are in the form of little masses of coloring-matter or pigment, 

 often of a strikingly bright color, enclosed in distinct cavities. 

 These are known as the " pigment-spots " or " eye-specks," 

 and they are believed to be rudimentary organs of vision. 

 They are placed in a conspicuous and unprotected position on 

 the margin of the disk, and hence these organisms were termed 

 " naked-eyed " Medusae, by Edward Forbes. The reproductive 

 organs are mostly developed in the course of the radiating 

 gastro-vascular canals, but are sometimes situated in the walls 

 of the central polypite. The above is the essential structure 

 of any of the ordinary naked-eyed Medusm ; and it is hardly 

 necessary to remark that it is exactly similar to what has 

 been formerly described as distinguishing the undoubted free- 

 swimming reproductive buds of the fixed Hydrozoa. The 

 probabilities, therefore, as before said, are in favor of the belief 



