60 THE ACALEPHAE. § 54. 



s 



en iiber die Scliirmquallen iiberhaupt, in the Mem. de I'Acad. des Sc. de 

 St. Petersburg, 6 ser. Tom. IV. 1838, j). 239. 



Milne Edwards. Observations sur divers Acalephes, in the Ann. des 

 Sc. Nat. 2de Ser. Zoologie. Tom. XVI. 1841, p. 194. 



ADDITIONAL BIBLIOGRAPHY. 



Forbes. A monograph of the British naked-eyed Medusae, with figures 

 of all the species. London, Ray Society, 1848. Contains many anatom- 

 ical details. 



Agassiz. Contributions to the Natural History of the Acalephae of 

 North America. 



Part I. — On the Naked-eyed Medusae of the shores of Massachusetts, 

 in their perfect state of development. 



Part II. — On the Beroid Medusae of the shores of Massachusetts, in 

 their perfect state of development. See the Mem. Amer. Acad. Arts and 

 Sc. vol. IV. 1850. 



Also, Twelve Lectures on Comparative Embryology, delivered before 

 the Lowell Institute, Boston, 1848-49. 



Busch. Beobachtungen iiber Anatomic und Entwickelung elniger wir- 

 bellosen Seetniere. Berlin, 1851. 



[The above are among the most important larger works; but see, also, 

 many papers of great value, to which I have referred in my notes. — Ew- 



TOJi.] 



CHAPTER I. 



SKIN AND CUTANEOUS SKELETON. 



§ 54. 



Generally, the body of the Acalephae is of a gelatinous substance, com- 

 posed of polyhedral cells. In some species certain parts of the body have 

 a cartilaginous hardness, but it is only in a few that there is found a carti- 

 laginous or calcareous nucleus, comparable to a rudimentary skeleton. 



With the Diphyidae a large portion of the body has a cartilaginous 

 density, and with the Physophoridae it is often surrounded by plates of a 

 similar nature. The Velellidae have a nuclear skeleton, which in Rata- 

 ria is a simple, elongated disc; but in Velella this disc, which is horizon- 

 tal and of an elongated oval form, is surmounted by a vertical crest. The 

 disc is composed of four pieces joined together by two sutures which cross 

 each other obliquely. The crest, united to the disc along the whole length 

 of the two sutures, and resembling the segment of a circle, is composed of 

 two main pieces, joined in the middle by a third, which is shaped like a 

 wedge. ^'^ 



The disc situated under the skin of the upper surface of Porpita, and 



•1 Eschsckoltz, loc. cit. Taf. XV. ; and Lesson, AcuMphes, loc. cit. PI. Xn. fig. 1; aXso, Duperrey, 

 Voyage loc. cit. Zoophytes, No. 6. fig. 1, A. A. 



