16 THE VOYAGE OF H.M.S. CHALLENGER. 
in many instances—especially in the true deep-sea species—the eyes are wanting (a 
matter to be discussed hereafter), it never happens that the tubercle has totally dis- 
appeared. Most genera have this tubercle placed nearly in the middle between the two 
ovigerous legs; but in some genera (Phowichilidiwm, e.g.) it is situated much nearer 
the front of the segment. 
The thoracic part of the cephalothorax and the three following true thoracic segments 
are furnished with lateral processes for the insertion of the legs; these lateral processes 
in the different genera, and even in different species of the same genus, are of very diffe- 
rent lengths. The segments of the body themselves are also of very different lengths. 
There are extremely slender forms with long segments and widely separated lateral 
processes, and there are also forms so highly concentrated, that the lateral processes 
are not separated at all; and between these extremes, which are often met with in one 
and the same genus, numerous intermediate forms are to be observed. The dorsal 
surface of the body is either smooth or furnished with knots, spines, strong prickles, &c. 
At its extremity, between the two lateral processes for the insertion of the last 
pair of legs, the last thoracic segment has a rudimentary abdomen of varying length, 
which is sometimes (Colossendeis, e.g.) connected with the segment by means of an 
articulation, and also sometimes (Zetes, Kréyer) shows traces of bemg divided into 
two segments.’ At its extremity the anal aperture is found. 
The legs begin at the ends of the lateral processes ; they are eight-jomted. For the 
joints I retain the names proposed by Johnston ; these names are the same as those used 
in entomology, but it is evident that in this case identity of name does not necessarily 
go along with identity of meaning; neither analogical nor homological comparison is 
meant by it. 
The first three (the coxal) joints are as a rule very short; the following three, the 
thigh and the two tibial joints, are much longer (the second tibial being in most cases the 
longest of all). The two tarsal joints are again a great deal ‘shorter. The first tarsal 
as a rule is shorter than the second; in many instances it is even extremely small, its 
function then being only to furnish a highly movable articulation to the last joint of the 
leg. At its extremity the last joint is furnished with a claw, which is, or is not, accom- 
panied by two accessory claws.” In some genera (Colossendeis, e.g.) accessory claws are 
never observed, while in other genera (Nymphon) they occur in some species and are 
wanting in others, Therefore the presence or absence of accessory claws alone should 
not be made use of in establishing new genera. 
1 Rhynchothorax mediterraneus, Cos., Microdoride mediterranea, Napoli, 1861, has a seven-jointed abdomen (Addome 
angusto e brevissimo di 7 articoli). 
? | think there is not a single reason for calling this claw a ninth joint. At any rate the homology of the claw with its 
accessory claws is much greater than that between the claw and the joints of the leg, and, therefore, if the claw is con- 
sidered as a ninth joint in those cases where accessory claws are observed, we must speak of a joint having two lateral 
joints close to its origin, which would be absurd. ‘ 
