ACALEPH^. 



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] 



ACANTHODIUM. 



case, it is situated in tlie middle of the under 

 side and leads into a stomach, which is fre- 

 quently furnished ^xiih caecal appendages. 

 When several oral apertu.res are present, 

 either several oesophageal canals conduct the 

 nutriment through the arms, in which the 

 oral apertures are placed, to a central sto- 

 mach, or each separate mouth is connected 

 with a distinct tubular stomach. A distinct 

 hepatic organ has not yet been found. 



The respiratory system consists of vessel- 

 like canals traversing the body, filled with 

 water, and either terminating in the gastric 

 cavity or opening externally. These are lined 

 internally mth ciha, and represent a water- 

 vessel system. 



The blood-vessel system consists of a set of 

 closed vessels with very delicate walls, ac- 

 companying and enclosing the water-vessels, 

 and containing a coloured liquid with co- 

 loured globules, representing the blood. But 

 there is no regular circulation; the indivi- 

 dual parts of the vascular system contract 

 here and there irregularly, whence the blood- 

 corpuscles become moved very slowly and in 

 no definite direction. Neither are the blood- 

 vessels lined with cilia. 



The Acalephse are propagated by the for- 

 mation of ova, and according to the plan of 

 alternation of generation. They are either 

 hermaphrodite or unisexual. 



The reproductive organs of the two sexes 

 frequently so closely resemble each other in 

 colour, external form and arrangement, that 

 they might easily be mistaken for each other, 

 without examination of then contents. They 

 form either utricular or strap-shaped stripes, 

 placed at various parts of the body. In the 

 former case, the spermatic fluid and the ova 

 are evacuated through distinct excretory ducts; 

 in the latter, the spermatozoa and ova escaping 

 from the strap-shaped testis or ovary, pass 

 directly outwards, or into capacious cavities, 

 opening externally by wide orifices. The 

 ova are round, and surrounded by a single 

 very delicate capsule, and the germinal ve- 

 sicle with its simple germinal spot is visible 

 through the whitish, violet or yellow yolks. 

 The spermatozoa move rapidly in, and are 

 unafi'ected by water ; they are sometimes li- 

 near, at others one end is rounded, the other 

 prolonged into a capillary appendage (PL 

 40. fig. 5*). 



The developmental metamorphosis of the 

 Acalephse (Medusa) is very remarkable. 

 When the ordinary process of segmentation 

 of the entire yolk is completed, the ova be- 

 come converted into ovate infusoria-like em- 



bryos (PI. 40. fig. 6), which revolve upon 

 their longitudinal axis by means of ciliated 

 epidermis, and swim about like species of 

 Leucophrys or Bursaria. After a time, they 

 become fixed at the anterior extremity to some 

 body ; arms then shoot out from the unat- 

 tached extremity, between which the mouth 

 of the polype-like animal is developed 

 (PL 40. figs". 7 & 8). At this stage of de- 

 velopment the larvae multiply by the forma- 

 tion of gemmae (PL 40. fig. 9 a), and off- 

 sets or stolons (PL 40. fig. 9 h) ; and ulti- 

 mately undergo transverse division, which 

 takes place as follows : — the larvse grow in 

 length, and the body becomes constricted into 

 several segments, from each of which eight 

 bipartite processes shoot out in a whorl 

 (PL 40. fig. 10). The segments of the body 

 then separate from each other seriatim, from 

 before backwards, swim about with eight rays, 

 and at last become gradually developed into 

 perfect Medusae. Many of the Medusae are 

 phosphorescent, and render the sea luminous. 



Bibliography. Eschscholtz, System der 

 Acalephen, Berlin, 1829; Will, Ho?^6B Ter- 

 gestincB, Sfc, 1844 ; Ehrenberg, Ahhandl. der 

 Berl. Akad. 1835; Ai-t. Acalephce, Todd's 

 Cycl. Anat. andPhys. (R. Jones); Ai't. Ovum, 

 id. (A. Thomson); Siebold, Lehr. d. Vergl. 

 Anat.; Owen, Lectures on Comparative Ana- 

 tomy ; Huxley,PAi/. Trcw^. 1849; Leuckart, 

 Siebold and Kolliker's Zeitschrift fiir Wiss. 

 Zool. Bd. 3, 1851 ; Lesson, Suites a Buffon 

 {Zoophytes Acalephes); Wagner, Icones Zoo- 

 tomicce. 



ACANTHACEiE.— The seeds of many 

 genera of this family are clothed with hairs 

 composed of hygroscopic cells, containing 

 unrollable spiral fibres or detached rings. 

 Among these ave,Aca7ithodium spicatum, De- 

 lile, Blejjharis, and Buellia formosa. Other 

 species and genera have the hygroscopic cells 

 destitute of internal fibre, as Ruellia litto- 

 ralis, Phaylopsis glutinosa, Barleria nocti- 

 flora, Lepidagathis, &c. Further particu- 

 lars respecting the hygroscopic cells will be 

 found under Cell-membrane and Spiral 

 STRUCTURES. See also Acanthodium and 

 Ruellia, and for a similar phaenomenon in 

 other families, Collomia,Cob^a, Salvia. 



BiBL. Kippist, On the existence of Sjnral 

 Cells in the seeds of Acanthacece. Linnean 

 Transactions, vol. xix. p. 65. 



ACANTHODIUM. (Flowering Plants, 

 fam. Acanthacece). — Mr. Kippist thus de- 

 scribes the appearances })resented by the hairs 

 upon the seed oi Acanthodium spicatum, De- 

 lile. The entire surface of the seed is clothed 



