VERMES. 587 



a protrusible muscular proboscis, as in Archiannelida. There is a coelome, 

 traversed by strands of connective tissue. An excretory system is present. In 

 D. gyrodliatus it consists ' of five pairs of intracellular segmental canals, each with 

 its own external aperture, and terminating internally in a flame-cell ' (Lang, quoting 

 Meyer, ' Polycladen,' Fauna, &c., des Golfes von Neapel, xi. p. 678). Flame-cells 

 have been detected in D. gigas and D. apatris. In the latter a network of fine 

 canals is said to exist beneath the integument, as well as larger ciliated canals. 

 The larger canals, however, are not present in the head. Korschelt observed one 

 of them to open on the ventral surface near the ovary. The sexes are separate. Males 

 and females are alike in D. gigas and D. vorticoides, dissimilar in JD. apatris. The 

 male of the latter is much smaller than the female ; its head has a single ring of cilia, 

 and the ventral surface of its body is uniformly ciliated ; it has no eyes, no digestive 

 tract ; a conical perforated copulatory organ lies within a cavity at the posterior 

 extremity of the non-segmented body. The ovary is single in D. apatris, and is 

 attached to the digestive tract where the stomach passes into the intestine ; the ova 

 are set free into the coelome as large female, and small male, ova, surrounded by a 

 substance which swells up when they are laid ; the birth opening is ventral, and in 

 front of the anus. Segmentation in JD. apatris is unequal ; the gastrula epibolic ; 

 and the first stages of the development of the male resemble those of the female. In 

 JD. gigas the testis and ovary are Y-shaped masses of cells, situated as in D. apatris. 

 The genital products collect in the coelome, and probably destroy the parents by 

 rupture. Two or four ovaries are stated to exist in other species. 



Dinophilus is an interesting genus. It is probably to be regarded as a modified 

 Trochosphere, differing very markedly from the modification which gave origin to 

 the Rotifera. 



D. apatris (with lit.), Korschelt, Z. W. Z. xxxvii. 1882. D. gigas, Weldon, 

 Q. J. M. xxvii. (i), 1886. 



2. Gasterotricha. This group of minute organisms is defined by Ludwig as 

 follows : ' Small vermiform animals, with a well-defined ventral surface. Digestive 

 tract straight, divisible into an anterior muscular, and a posterior cellular, region. 

 Mouth and anus ventral. Body clothed with a cuticula, which bears processes 

 differing in character. Cilia, as a rule, confined to the ventral aspect, rarely 

 covering the region of the head. Posterior end of the body, as a rule, forked. No 

 nervous system recognisable. Hermaphrodite ? Reproduction by means of 

 summer and winter ova ; no metamorphosis.' Eyes may be present. The female 

 aperture lies dorsally in front of the forked end of the body ; the ova are set free 

 into the coelome. The distinction between summer and winter ova appears to be 

 known only in Ichthydium (Chaetonotus) larus. The freshwater genera are Ichthy- 

 dium (which, according to Ludwig, includes Chaetonotus\ Chaetura, Cephalidium, 

 Dasydites, and the marine genera Turbanella and Hemidasys. 



Ludwig, 'Gasterotricha,' Z. W. Z. xxvi. 1876. Chaetonotus, Butschli, ibid. 

 Hemidasys, Claparede, A. Sc. N. (5), viii. 1867. 



3. Echinoderidae. Marine. Minute, with a cylindrical body and flattened 

 ventral aspect. The body is segmented into eleven to twelve chitinoid rings, of 

 which the first or head is invaginable, and is armed at its anterior margin with four 

 rows of recurved hooks ; the second is provided with twelve longitudinal chitinoid 

 ribs, and closes over the first when invaginated; the third and fourth are entire, 



