ACALEPH^. 



107 



Cydippe. 



from a single small ganglion, which is closely connected with a 

 coloured eye- speck, situated at the middle of the superior extremity 

 of the body. 



The principal locomotive organs in the Beroe consist of unusually 

 large cilia, aggregated in lamelliform groups (^Jigs. 54 and 55, c c), 



which seeming plates are arranged, like 

 the paddles of a propelling wheel, along 

 eight equidistant bands, extending along 

 ^he surface from near one end of the 

 body to near the other. 



The organs by which the Beroe can 

 attach itself to, or poise its body on, a 

 solid surface, are the two long tentacles 

 which are fringed with spiral cirri. 

 These tentacles can be entirely with- 

 drawn into the two cavities g, g, which 

 extend along each side of the slender intestine /. This is continued 

 from the simple elongated vertical digestive sac (e), the form of 

 which, in transverse section, is shown in Jig. 55. The Acalephse, 

 which, like the Beroe, swim by the action of vibratile cilia, are termed 

 " Ciliogrades." 



The Physalia or Portuguese man-of-war has a large air-bag to aid 

 its swimming ; the Physophora floats by many smaller air vesicles : 

 the species so provided are called " Hydrostatic Acalephae." 



Two genera of Acalephse have an oval or circular gelatinous body 

 supported by an internal solid, cartilaginous, or albuminous plate : 

 numerous extensile tentacles or cirri depend from the under surface 

 of the body, in the centre of which is the mouth. These form the 

 order " Cirrigrades." There is no evidence, however, that they swim 

 by any action of their prehensile cirri. One of the genera, Velella, 

 has a process of the firm internal skeleton, rising from the upper sur- 

 face of the body- disc or deck, to which it is set at the same angle as 

 the lateen-sail of the Malay boat : it is wafted along by the action 

 of the wind upon this process, and may have been mistaken for the 

 fabled Cephalopodic paper-sailor (^Argonautd). 



The generative system and the development of the ovum in the 

 Medusae have received very satisfactory elucidation by the observa- 

 tions of Gaede, Cuvier, Ehrenberg, Sars, Siebold, and Wagner. 



Propagation by gemmation has been observed in the pulmograde 

 genus Cytaeis. An incomplete gemmation takes place in Stephanonia 

 and others ; but this simple vegetative mode of propagation, which 

 is so common in the lowest Infusories and Polypes, is here the ex- 

 ception. 



The Medusae are highly prolific, and propagate in the ordinary 



