ECHINODERIDAE. 311 



often prolonged into spines and hairs, a cerebral ganglion connected with the 

 ectoderm and with mouth and anus. 



The Gastrotricha (Fig. 250) are small fresh-water organisms of unknown 

 affinities. They have a ventral mouth in front, a dorsal anus behind, and an 

 alimentary canal presenting a muscular pharynx, stomach lined withj^ few large 

 cells, and a rectum. The body cavity is indistinct, and if present is without 

 epithelioid lining. There is no vascular system, and the excretory organs 

 consist of two coiled tubes opening on the ventral surface, and internally ending 

 in a ciliated portion. The ovaries are paired, and no oviducts have been observed. 

 It is doubtful if they are hermaphrodite, and if a small median organ behind the 

 ovaries is a testis. No male is known. There is only one kind of egg, found in 

 summer as well as winter. The muscular system is highly developed, and 

 in part in specialized muscular bands. There are no transversely arranged 

 muscular fibres. The cilia are in two ventral rows, and a patch is found on 

 the head in the neighbourhood of the mouth. There is a dorsal ganglion in 

 front which extends a little distance backwards on each side. 



The marine forms, Hemidasys Clap, and Turbanella M. Sch., appear not to 

 be Gastrotricha. The former has a parenchyma, a well-developed testis, vas 

 deferens, and chitinous penis, and two small ciliated pits in front like Nemer- 

 tines and some Turbellarians ; the ovary, nervous system, and excretory organs 

 are unknown. In Turbanella the whole ventral surface is ciliated, and there is 

 no chitinous cuticle ; and there are two ciliated pits. 



Section 1. Euichthydina. 



With forked tail, and adhesive glandular apparatus. 



Fam. 1. Ichthydiidae. Skin either naked or beset with scales or papillae, 

 never with spines. Ichthydium Ehbg. ; Lepidoderma Zelinka. 



Fam. 2. Chaetonotidae. Skin either with spines or with spines placed on 

 scales. Chaetonotus Ehbg. ; Chaetura Metschn. 



Section 2. Apodina. 



Without caudal fork ; hind end either simply rounded, or lobed and provided 

 with tufts of hairs. Dasydytes Gosse ; Gossecc Zelinka. 



ECHINODERIDAE.* 



These are minute marine animals with an elongated body, covered with a strong 

 cuticle which is divided into segments, and an anterior protrusible spiniferous 

 proboscis, at the apex of which the mouth opens. The anus opens at the hind 

 end of the body. The cuticle is produced into bristles and spines. The central 

 nervous system, which lies in the ectoderm, consists of a nerve-ring surrounding 

 the pharynx and clothed with nerve-cells, and of a ventral cord with segmentally 

 arranged groups of ganglia. The alimentary canal consists of a muscular pharynx, 

 a non-muscular intestine, and a rectum. The excretory organs are represented 

 by blind ciliated tubes, which open separately on the dorsal surface. The 

 animals are dioecious ; the testes and ovaries are paired tubes opening ventrally 

 on either side of the anus. They occur in mud on the sea-bottom, and on marine 

 Algae. 



* W. Reinhard, " Kinorhyncha (Echinoderes), anat. Bau u. Stellung ini 

 System," Z. f. w. Z., 45, 1887. C. Zelinka, " Ub. Echinoderes," Verh. d. 

 deutschen Zool. GcseL, 1894. 



