146 CRUSTACEA. 



FAMILY II. 



SIPHONOSTOMA. 



The Siphonostomse have no kind of jaws whatever. A 

 sucker or siphon, sometimes external and in the form of an 

 acute inarticulated rostrum (1), and at others concealed or 

 but slightly visible, fulfils the functions of a mouth. There 

 are never more than fourteen feet. The shell is very thin 

 and composed of a single piece. They are all parasitical. 



We will divide this family into two tribes. 



The first Caligides, Lat. is characterized by the pre- 

 sence of a shell resembling an oval or semi-lunar shield ; by 

 the number of visible feet, which is always twelve, or four- 

 teen, if we include those which Leach considers as such, and 

 which I call inferior antennae ; by the form and size of the 

 tenth pairs which are sometimes multifid, pinnate or ter- 

 minated in a fin, and well adapted at all times and in the 

 adult, for the purposes of natation, and sometimes foliaceous, 

 or broad and membranous. The sides of the thorax are never 

 furnished with wing-like expansions directed backwards and 

 enclosing the body posteriorly. 



Here, the body, exhibiting several segments above, is 

 elongated and narrowed posteriorly, terminating in a kind of 

 tail with two threads or as many other salient appendages at 

 the end; this extremity is not covered by a segment of the 

 superior teguments in the form of a large rounded scale, 

 deeply notched in the posterior margin. The shell is at 



(1) The composition of this rostrum or beak is not well known. It is evident, 

 from the figure of the Jlrgulus foliaceus, given by Jurine, Jun., that it contains a 

 sucker; but is this the case with the others, and of how many pieces is it com- 

 posed? I cannot answer the question. I presume, however, that this siphon con- 

 sists of the labrum, mandibles and the ligula which forms the sheath of the sucker. 

 In the preceding Entomostraca, the four anterior feet, whose form is very differ- 

 ent from that of the following ones, would correspond to (he four jaws of the De- 

 capoda. 



