PYCNOGONOIDEA. ^385 



The following organs in the Caligoidea are legs. The mouth organs 

 hence correspond to but two normal segments, the mandibular and 

 maxillary. The legs are twelve in number, and with the preceding, 

 make in all sixteen appendages. 



For farther comparison, we observe, that in the Cyclopoidea, the 

 mouth organs include two pairs of organs, besides the mandibles, 

 and the legs are ten or twelve in number, making in all sixteen or 

 eighteen. In the Cyproidea, the number of appendages counting from 

 the mandibles is ten. In Limulus, the number is ten, besides five 

 thoracic lamellae behind, corresponding to five additional pairs. In 

 the Pycnogonoids, the proboscis appears to correspond to the buccal 

 trunk of Caligus ; and all that exists of the anterior segment of the 

 body, or the head if we may so call it, is the small segment bearing 

 the eyes. If, therefore, we should cut out a small medial portion 

 from the Caligus, so as to keep the eyes and trunk, and perhaps some 

 adjoining appendages, we should have, in some respects, a representa- 

 tive of the Pycnogonum structure, which would be rendered more 

 complete by elongating the trunk, and reflexing the segment, so as to 

 make the trunk terminal upon the head.* 



The anterior appendages of the cephalic or ophthalmic ring next 

 demand consideration. As this segment bears the trunk as an appen- 

 dage, instead of being a posterior ring, its appendages are not neces- 

 sarily posterior to the mouth in their normal relations. On the con- 

 trary, it is quite as probable, that the first pair of organs may be nor- 

 mally antennae, anterior to the mouth or the mandibular ring. In 

 fact, they are often somewhat higher in position, rising more nearly 

 from the upper part of the segment, as if of this character, and this 

 view is sustained by various considerations. The prehensile form is 

 in favour of it; for it is the prevailing form throughout the sucking 

 Crustacea, as well as in other species, as the Corycaai, Sapphirinse, and 

 Limuli.f This then is no objection. In Dichelestion, which has the 

 narrow articulated body almost of a Pycnogonoid, although of another 

 type in its legs, these organs project in front, and are ancoral. 



The study of the young or embryonic forms of the species gives 



* The usual coalescence of the cephalic segment with the first thoracic segment finds 

 an analogy in the genera Tanais, Caprella, and many others. 



■j- No organs undergo wider variations of structure ; they may be legs, hands, oars, or 

 simply antennae, according to the group : and in most of the Entomostraca they are in 

 some way used for attachment or for prehension. 



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