230 SP1NTHER ONISCOIDES. 



reaches the length of 50 ram. A general structural description follows, and he points out 

 the essential differences of the dorsal lamella) in the three species. Bifid dorsal bristles 

 only are present in S. miniaceus, while in the other two simple bristles accompany the 

 former. The mouth opens on the ventral surface anteriorly, and four small brown eyes 

 lie at the base of the tentacle, each with a short thick nerve from the cephalic ganglion. 

 The nerve-cords are separate except for the transverse commissures. The alimentary 

 canal consists of fore, middle, and hind gut, besides caecal appendages. The pharynx is 

 muscular. The mid-gut possesses generally a pair of diverticula in each segment, and 

 terminates in an anus which has a pair of short cirri. Excretory organs are absent, but 

 a blood-system is present. There are no differentiated branchia3. The reproductive 

 elements are separated from the body-wall, and lie in the body-cavity. All are parasitic 

 on sponges. 



Recently Racovitza 1 (1896) has carefully investigated the structure and homologies 

 of the cephalic region, and he is of opinion that only the sincipital region of the head 

 remains with the tentacle and the four eyes, and the much reduced brain internally ; the 

 latter (brain) consisting of the middle region only, whereas in Eurythoe' and Euphrosyne 

 both the anterior and posterior regions of the brain are present. This reduction of the 

 cephalic region he attributes to parasitism on Esperella and other sponges. 



The species show a certain amount of variation, and von Graff thinks that the some- 

 what radiate arrangement of the parts anteriorly and posteriorly approaches the condition 

 in Myzostoma, which occurs on starfishes, while Spinther frequents sponges. 



In the pinnate arrangement of the alimentary caeca Sjpinther differs from the typical 

 Euphrosynina, and approaches the Aphroditidse. 



1. Spinther oniscoides, Johnston, 1845. 



Specific Characters. — A dorsal cirrus at the end of the parapodium. The dorsal 

 lamella project at the free margins and have a series of strong, simple or bifid bristles. 

 The hooks of the parapodia are powerful, pectinate at the projecting margin, and the tip 

 is strongly curved. Ventral surface furnished with small papillae (warts) on the ridges 

 which flank a median furrow. Posterior end split to the anus. 



Synonyms. 



1845. Spinther oniscoides, Johnston. Ann. Nat. Hist., vol. xvi, p. 8, pi. ii, f. 7 — 14. 



1854 u Gryptonota citrina, Stimpson. Marine Invest. Grand Marian, p. 35, pi. ii, f. 27. 



1865. Spinther oniscoides, Johnston. Cat. Brit. Mus. ; p. 127, pi. xiv, f. 7 — 14. 



1882. „ arcticus, Hansen. Annel. Norsk. Nordhavs-Exped., p. 44, pi. i, f. 1 — 5. 



1883. „ major, Levinsen. Syst. Oversigt. nordiske Annulata, &c, Yidensk. Meddelels, p. 125. 

 1887. „ oniscoides, Graff. Zeitsch. f. w. Zool., vol. xlvi, p. 90, pi. vi — vih, and woodcuts. 



Habitat.— Dredged off Castle Chichester, Belfast Bay, by Mr. Hyndman in 1844, 

 and off Anglesey in seventeen fathoms (Hornell) ? (fide Dr. A. Merle Norman). Abroad 

 1 < Arch. Zool. Exper./ 3 ser., vol. iv, p. 197, &c, 1896. 



