ACALEPH^. 



[ 2 ] 



ACALEPH.E. 



otliers aggregated into groups. The paler 

 and more delicate coloiu's are said to arise 

 in some instances from pigment uniformly 

 dissolved in the substance of tlie body ; it 

 is most probable, however, that they arise 

 from iridescence. 



Muscular sr/sfem forming long, thin, reti- 

 cular fibres and bundles, almost everywhere 

 penading the contractile substance of the 

 body. 



The floating and locomotion of these ani- 

 mals are often aided by larger or smaller 

 cavities filled with air. 



The nervons system consists of a ring 

 following the margin of the disk, with 

 ganglionic expansions at intervals, giving 

 off branches to the tentacles and the radial 

 canals. 



The orc/ans of sense consist of tubercular 

 or spathulate bodies situated near the mar- 

 gin of the body or at the base of the ten- 

 tacles, and connected with adjoiningganglia. 

 Some of these are regarded as organs of 

 vision (ocelli) ; and consist essentially of a 

 membranous capsule containing a clear 

 liquid with cr^-stals of carbonate of lime, 

 and sometimes a red or black pigTuent 

 (PI. 49. fig. 5 //). Those which contain no 

 pigment have been considered to be of 

 auditoiy function, and the crystalline bodies 

 otolithes. Some of them are protected by 

 an overhanging fold of membrane ; hence 

 the distinction of covered- and naked- eyed 

 Medusa- ; but the latter are now regarded as 

 the sexual zooids or gonophores of the 

 Ilydroida (THArMANTiAs). 



The digestive cavity, which is situated in 

 the middle of the body, is lined with cili- 

 ated epithelium and furnished with distinct 

 walls (endoderm), which are directly con- 

 tinuous with the general parenchyma of the 

 body, so that there is no abdominal cavity. 

 The mouth is either single and central, or 

 multiple. In the former case, it is situated 

 at the end of the peduncle, in the middle of 

 the under side, and leads into a stomach, 

 which is frequently furnished with cacal 

 appendages. "When several oral apertures 

 are present, either several cesojdiageal canals 

 conduct the nutriment through the arms, 

 in which the oral apertures are placed, to a 

 central stt>mach, or each separate mouth 

 is connected with a distinct tubular sto- 

 mach. A distinct hepatic organ has not 

 yet been found. ]Jay Lankester dcscriles 

 the inception of natural food-materials in 

 the cells of the endodeim. 



GastrovascuJar system. A number of 



But there is no regvdar circu- 



vessel-lilce canals run from the stomach or 

 central cavity throughout the body, the 

 principal branches forming rays from the 

 centre to the margin, communicating finally 

 with a circular vessel traver.~ing its circum- 

 ference (PL 49. fig. 5 d). These are also 

 Imed with cilia, and contain both the focd 

 and water 

 lation. 



The Acalephae are propagated by the for- 

 mation of ova, and according to the plan of 

 alternation of generations. They are either 

 hermaphrodite or xmisexual. 



The reproductive organs of the two sexes 

 are often so similar in colour, external form, 

 and arrangement, that they might easily 

 be mistaken for each other, without exami- 

 nation of their contents. They forni either 

 utricular or strap-shaped stripes, placed at 

 various parts of the body, often near the 

 rays of the gastrovascular system. In the 

 former case, the speiTuatic fiuid and the ova 

 are evacuated through distinct excretory 

 ducts ; in the latter, the spermatozoa and 

 ova escaping fi-om the strap-shaped testis 

 or ovary, pass directly outwards, or into 

 capacious cavities opening externally by 

 wide orifices. The ova are round, and sur- 

 rounded by a single very delicate capsule ; 

 and the genninal vesicle with its simple 

 germinal spot is visible through the whitish, 

 violet, or yellow yolks. The spermatozoa 

 move rapidly in, and are imaft'ected by 

 water ; thej are linear, or one end is 

 rormded, the other prulonged into a capil- 

 lary appendage (PI. ^9. fig. 5*). 



The developmental metamorphosis of 

 some of the Acalepb.fe is very remarkable. 

 When the ordinary process of segmentation 

 of the entire yoUc is completed, the ova be-r 

 come converted into ovate infusoria-like 

 embryos or planida? (PI. 49. fig. 6), -\\ hich 

 revolve upon their longitudinal axis by 

 means of cihated epidermis, and swim about 

 like species of Levco}>hrys or Bursaria. 

 After a time, they become fixed at the 

 anterior extremity to some body; arms 

 then shoot cut from the imattached ex- 

 tremity, between which the mouth of the 

 polvpe-like animal (Hvdra-tuba state) is 

 developed (PI. 49. figs'. 7 & 8). At this 

 stage of development, the larvas multiply by 

 the foimation of gemmae (PI. 49. fig. 9 o), 

 and cffsets or stolons (PI. 49. fig. 9 h) ; and 

 ultimately each undergoes transverse di- 

 Aision, which takes place as follows: — the 

 larvse grow in length, and the body be- 

 comes constricted into several segments 



