CRUSTACEA. 



A striking case of this occurs in the lobster, which viewed from above shows the an- 

 terior portion of the body covered by a large shield, the carapax, with a transverse 

 line. On careful study it is found that this anterior portion, to anticipate a little our 

 account, represents the dorsal portions of the mandibular and second antenna! segments, 

 and the impressed line represents the nearly obliterated suture between the two 

 somites. This carapax covers more than the two segments mentioned, and if the seg- 

 ments covered are examined it will be found that the rings are not complete, the 

 upper portion being absent, its place being supplied by the carapax. It is frequently 

 attempted to divide the anterior portion of the body of the Crustacea into head and 

 thorax, corresponding to the same portions of insects, but without any great success, as 

 in nature no such division exists, and the attempts to homologize the appendages of the 

 insects with those of the Crustacea are never productive of any very satisfactory 

 results. Still, the divisions are very convenient, and in this 

 work will be used with functional limits, the head containing 

 those segments connected with the senses and with eating, the 

 thorax, those whose appendages are principally locomotive, while 

 the abdomen embraces the segments behind the thorax. Thus, 

 in the Decapoda the line between head and thorax will come 

 between the eighth and ninth segments ; in the Tetradecapoda 

 between the sixth and seventh, etc. 



Returning now to the limbs or appendages which we left 

 for the moment, we find other things for consideration. Besides 

 the two branches, exopodites and endopodites, which we have 

 mentioned, the limbs frequently bear a third, the epipodite or 

 flabellum, and the gills or respiratory organs, of which more 

 anon. The form of the limbs is subject to great variation 

 according to the functions they have to perform, some being 

 the supporters of organs of sense, those around the mouth taking 

 part in the capture and preparation of food, the next group in 

 walking or swimming, while still others play a subordinate part 

 in the perpetuation of the species, either as intromittent organs 

 or as supporters of the eggs. The two anterior pairs of appen- 

 dages, the antennulce and antennae are always in advance of the 

 mouth, and seem to be chiefly sensory, the ear in many forms 

 occurring on the basal joint of the antennula. The mandibles, 

 the first pair of appendages behind the mouth, come next, and 

 are succeeded by two pairs of maxillce, and then the maxillipeds, 

 of which in the Tetradecapoda there is but a single pair, in the 

 Decapoda three pairs. Beyond this point in any account of the 

 limbs of the Crustacea as a whole it is difficult to go, and the 

 reader is referred to the portions upon the special groups for in- 

 formation upon this point. 



Strange as it may at first sight seem, a true heart is not in- 

 variably present in all Crustacea ; in certain forms the blood is 

 propelled merely by the motions and consequent changes in the 

 shape of the body, and no central propelling organ exists. When 

 present, the heart consists either of an elongate tube or a short sac directly beneath 

 the integument of the back. From the heart the blood is carried in arteries to 



FIG. 8. Internal structure 

 of lobster, half natural 

 size. b. Brain, e. Eye. g. 

 Gills, h. Heart, ha. He- 

 patic artery, i. Intestine. I. 

 Liver, m. Muscles, o. Oph- 

 thalmic artery, s. Stom- 

 ach, pa. Posterior artery. 



