310 THE ANATOMY OF INVERTEBRATED ANIMALS. 



erately elongated, and situated in the posterior part of the 

 thorax. 



Dohrn 1 has shown that the development of the Cumacea 

 takes place without metamorphosis. In most respects the 

 embryo resembles that of My sis but, instead of the cuticu- 

 lar investment of the transitory JVauplius-stage with its two 

 pairs of appendages, there is only a sort of cuticular sac with 

 a thickening in the middle line of the tergal aspect, which the 

 embryo bursts as it acquires a larger size. In this respect, 

 the resemblance of the embryonic development of the Cuma- 

 cea to that of the Edri ophthalmia is, as Dohrn points out, 

 very striking, and no doubt they form a connecting-link be- 

 tween the Podophthalmia and the Edriophthalmia. Having 

 regard to their whole organization, on the other hand, they 

 stand at the bottom of the Malacostracan group, and are com- 

 parable to a Peneus-\arva, in the Copepod stage, the limbs and 

 body of which are modified in the direction of the Schizopoda, 

 while the fore-part of the head has remained Copepodous. 



Fossil Brachyura are abundant in tertiary deposits, but 

 are rare in formations of earlier date. Macrura of a pecul- 

 iar type (Eryon) occur in the mesozoic rocks, and perhaps 

 the carboniferous Gampsonyx should be referred to the Po- 

 dophthalmia. 



THE EDRIOPHTHALMIA. These resemble the Podophthal- 

 mia in never possessing a greater than the typical number (20) 

 of somites, though, in some members of the group, the body 

 is composed of fewer somites, in consequence of the abortive 

 or rudimentary condition of the abdomen. Eyes may be 

 absent ; when present, they are usually simple, and are either 

 sessile or seated upon immovable peduncles (Munna). The 

 antennules almost disappear in the terrestrial Isopoda, while 

 the antennae become rudimentary or vanish in some Am- 

 phipoda. The mandibles lose their palps in the Woodlice ; 

 which thus, as in the presence of only one pair of well-devel- 

 oped aritennary organs, approach Insects. Ordinarily, the 

 posterior seven, and, at fewest, the posterior four, thoracic 

 somites are perfectly distinct from, and freely movable upon, 

 one another. The ophthalmic and antennary somites have 

 coalesced with the rest of the head ; the branchiae depend 

 from the thoracic limbs, or are modifications of the abdomi- 

 nal appendages ; and the heart is elongated and many-cham- 



1 " Ueber den Bau und die Entwickelung der Cumaceen." (" Untersuchun- 

 gen uber Bau und Entwickelung der Arthropoden," 1870.) 



