58 ARENICOLA. 



secreted by the cyst. In Arenicola Grubei and A. ecaudata the closed otocysts are 

 devoid of vibratile cilia, and contain one large otolith and several smaller— all secreted 

 by the otocyst, and only having Brownian movements. In all the species the otocysts 

 (in the peristomium) are innervated from the oesophageal collar. 



Downing (1911) 1 observes that in Arenicola crlstata the spermatophore arises as the 

 resnlt of cleavage of a primary spermatogonium. It is shed into the coelom even before 

 the cleavage of the spermatogonium has begun, becoming a hollow mass of cells which by 

 false invagination becomes gastrula-like, then flattens out — a saucer-like mass of sper- 

 matids. He suggests that the spermatophore is an individual, the gametozoon, which 

 bears the gametes ; the adult male is a sporozoon, which develops the spore or primary 

 spermatogonia; and he holds that it is a kind of alternation of generations. 



The Arenicolidse were entirely absent from the collections of the ' Challenger,' and 

 this of itself more or less defines the distribution of the group to comparatively shallow 

 water. 



Sub-Family I. — Arenicolid^ caudata, Ashivorth. 



Head (prostomium) has a median and two lateral lobes ; brain with distinct anterior, 

 middle, and posterior regions. Body often enlarged anteriorly. Feet and gills do not 

 extend to the posterior end — leaving a distinct tail. Gills pinnate or derivable therefrom, 

 eleven to thirteen pairs, the first (which may be small or absent) on the seventh or eighth 

 setigerous segment. Dorsal lip of segmental funnel (nephrostome) with numerous, 

 flattened, spathulate, ciliated, branched vascular processes ; ventral lip ciliated, entire. 

 Gonads small, ova discoidal ; thin zona. Segmental organs six pairs in segments four to 

 nine, or five pairs in segments five to nine. 



Genus XC. — Arenicola, Lamarck, 1801, char, emend. 



Liimbricns, Linna3us ; Chorizobranchus, De Quatrefages; Clymenides, Claparede. 



Cephalic lobe small, conical, with a single palpode, or provided with two rudimentary 

 palps, often with statocysts. Black granular pigment in the hypoderm (epiderm, 

 Fauvel) ; anterior diaphragms ; post-buccal segments two and three uncinigerous ; pair of 

 oesophageal coeca ; two lateral hearts each formed of auricle and ventricle. Branchial 

 segments of five annuli ; gills much branched, borne on the setigerous ring, fixed to the 

 dorsum. Dorsal division of the feet tubercular, arising anteriorly at the inferior face of 

 the stomachal sinus, posteriorly of the dorsal vessel. Inferior division of the foot forming 

 a torus. Dioecious : gonads borne only on the segmental organs. 



Fauvel (1899) groups the forms specially considered here thus : No achsetous abran- 

 chiate caudal region ; eleven to twelve uncinigerous abranchiate segments — A. branchlalis 

 (Grubii) ; fifteen to sixteen uncinigerous abranchiate segments — A. ecaudata. Prostomium 

 of three sub-equal lobes, first pair of branchiaa small— A. marina. The same author 2 



1 ' Science/ vol. xxxiii, p. 272. 



2 < Compt. Rend, Acad. So./ Paris, t. cxxix, p. 1273, 1899. 



