ACALEPELE. 



ACALEPH.E. 



others aggregated into groups. The paler 

 and more delicate colours are said to avi>t i 

 in some instances from pigment uniformly 

 dissolved in the substance of the body ; it 

 is most probable, however, that they arise 

 from iridescence. 



Muscular system forming long, thin, reti- 

 cular fibres and bundles, almost everywhere 

 pervading 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 nervous 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 organs 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 adjoining ganglia. 

 Some of these are regarded as organs of 

 vision (ocelli) j and consist essentially of a 

 membranous capsule containing a clear 

 liquid with crystals of carbonate of lime, 

 and sometimes a red or black pigment 

 (PL 49. fig. 5 ff). Those which contain no 

 pigment have been considered to be of 

 auditory 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 

 Hydroida (THATJMANTIAS). 



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 caecal 

 appendages. When several oral apertures 

 are present, either several cesophageal canals 

 conduct the nutriment through the arms, 

 in which the oral apertures are placed, to a 

 central stomach, or each separate mouth 

 is connected with a distinct tubular sto- 

 mach. A distinct hepjatic organ has not 

 yet been found. Ray Lankester describes 

 the inception of natural food-materials in 

 the cells of the endoderm. 



Gastrovascular system. A number of 



vessel-like 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 traversing its circum- 

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

 lined with cilia, and contain both the loud 

 and water. But there is no regular circu- 

 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 unisexual. 



The reproductive organs of the two sexes 

 are often so similar in colour, external form, 

 and arrangement, that they might ca-ily 

 be mistaken for each other, without exami- 

 nation of their contents. They form 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 spermatic fiuid 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 sur- 

 rounded by a single very delicate capsule ; 

 and the germinal vesicle with its simple 

 germinal spot is visible through the whitish, 

 violet, or yellow yolks. The spermatozoa 

 move rapidly in, and are unaffected by 

 water j they are linear, or one end is 

 rounded, the other prolonged into a capil- 

 lary appendage (PI. 49. fig. 5*). 



The developmental metamorphosis of 

 some of the Acalephae is very remarkable. 

 When the ordinary process of segmentation 

 pf the entire yolk is completed, the ova be- 

 come converted into ovate infusoria-like 

 embryos or planulae (PL 49. 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 bcdy ; arms 

 then shoot out from the unattached ex- 

 tremity, between which the mouth of the 

 polype-like animal (Hydra-tuba state) is 

 developed (PL 49. figs. 7 & 8). At this 

 stage of development, the larvae multiply by 

 the formation of gemmae (PL 49. tig. 9 ), 

 and offsets or stolons (PL 49. fig. 9 b) ; and 

 ultimately each undergoes transverse di- 

 vision, which takes place as follows : the 

 larvae grow in length, and the body be- 

 comes constricted into several segments 



