THE PODOPHTHALMIA. 293 



After the liver, genitalia, and antennary glands are de- 

 veloped, the yelk-sac eventually becomes reduced to a small 

 cascal diverticulum, situated at the pyloric end of the stom- 

 ach. The genital ducts in both males and females are origi- 

 nally diverticula from the corresponding regions of the geni- 

 tal glands ; their external apertures and the copulatory ap- 

 pendages of the first abdominal somites in the male are not 

 developed until some time after birth. 



The modifications of structure observable within the limits 

 of the Pod 'ophthalmia are exceedingly interesting. 



Excluding, for the present, the JSqiiiUidce, the group is 

 divisible on clear morphological grounds into the following 

 subdivisions : 1. The Prachyura y 2. The Anomura y 3. The 

 Macrura y 4. The Schizopoda. 



The morphological relations of the Macrura are nearly 

 such as are indicated by their position in this series ; and 

 Astacus, as a central genus of the central group, thus be- 

 comes a sort of natural centre for the whole of the Podoph- 

 thalmia, whence we may trace a gradual series of modifica- 

 tions, leading on the one hand to the /Schizopoda, with their 

 large abdomen and small cephalo-thorax ; and on the other to 

 the Prachyura, with their rudimentary abdomen and com- 

 paratively enormous cephalo-thorax. 



In all the Macrura the branching are numerous, and are 

 covered by the branchiostegites. The abdomen is large, and 

 is used as a locomotive organ, the appendages of its sixth 

 somite being well developed. The thoracic ganglia usually 

 form an elongated chain, and the external maxillipedes never 

 form broad opercular plates over the other jaws. In some of 

 the lower Macrura (Peneus, Pasiphma) the exopodite per- 

 sists as an appendage at the base of the thoracic limbs ; and 

 in two genera, Sergestes and Acetes, the posterior thoracic 

 members become^ rudimentary, or even entirely abortive, 

 though the abdominal appendages remain. 



In the higher Macrura, such as Palinums, the nervous 

 system exhibits a greater degree of concentration, the tho- 

 racic ganglia constituting an elongated oval mass ; and it is 

 in this genus and its allies that the head and its appendages 

 exhibit modifications, which prepare us for those which are 

 presented by the Prachyura. In this respect the Palinurus 

 vulgaris (Rock Lobster, Sea Crayfish, or Spiny Lobster) is 

 particularly worthy of attention. The rostrum is rudimentary 

 and represented by a mere spine, leaving the anterior cephalic 



