464 REPORT OF COMMISSIONER OF FISH AXD FISHERIES. 



they are referred to the Crustacea, by some to the Arachnida, while 

 others place them in a group distinct from both. In some respects they 

 must be regarded as intermediate between these groups ; and hence 

 to trace their homologies, especially those of the appendages, is a 

 matter of considerable difficulty. Some of the hairy species bear a close 

 general ftsemblance to spiders, which has given rise to their common 

 name of sea-spiders. Their anatomical structure is, however, very differ- 

 ent from that of the spiders, and in theii- sluggish movements and para- 

 sitic habits they are still more unlike those active and predacious ani- 

 mals. Most of the species cling to other animals, such as sponges, sea- 

 anemones, and particularly tubularian and other hydroids ; upon these 

 animals they i^robably in part feed, sucking their juices by means of the 

 large proboscis or rostrum, though their food apparently consists also 

 of more solid matters. They are remarkable, as a whole, for the reduc- 

 tion of the abdomen, and the great development of the legs, which some- 

 times have an extent equal to nine or ten times the length of the body ; 

 the abdomen is always aborted, so as to often appear like a mere tuber- 

 cle, and, with the exception of one or two forms where it is bi-articulate, 

 it is not divided into segments. The body shows exteriorly four seg- 

 ments, exclusive of the rostrum and abdomen ; these segments expand 

 laterally into prominent processes, which may readily be mistaken for the 

 basal joints of the legs, to which they give attachment. The abdomen 

 arises from the posterior segment, from which it is not, as a rule, sepa- 

 rated by segmentation. It is usually directed more or less upwards ; 

 at its extremity is the anus, usually in a deep cleft. 



The most anterior pair of appendages, which are wholly wanting in 

 a few forms [Pycnogonum, etc.), are here regarded as antennae, a view 

 which seems to me to be justified by their position and the origin of 

 their nerves; by many wiiters they are, however, considered to be post- 

 oral, and as probably representing mandibles. In the higher forms 

 they are three-jointed and usually forceps-like or 'chelate,' in other 

 genera two-jointed, and a recently described genus [Tanysti/lum), with 

 the antennae composed of a single joint, comi^letes the transition to 

 those forms in which antennae have quite disapijeared. It may be here 

 mentioned that antennae are invariably present in the larva, so far as 

 known; and that they are then alvv'ays three-jointed and chelate, their 

 subsequent disappearance in certain forms being apparently a case of 

 "retrograde development.'' Below the antennae is the large proboscis 

 or rostrum, at the extremity of which is the mouth ; this is triangular 

 in shape, and is sometimes fuiniished with three denticulated organs not 

 very unlike the jaws of a leech. Withm this rostrum is a large cavity, 

 continuous posteriorly with the oesophagus, and containing a compli- 

 cated apparatus for masticating food; this consists of a great number 

 of chitinous bars lying transversely in the walls of the cavity and giving 

 attachment to numerous setae, usually bifid at their tips, which extend 

 forward toward the mouth. Posterior to these are found in some spe- 



