6 Prof. Allman on a new genus and species of Entomostraca. 



succeeded in detecting. Be this as it may, the creature has now 

 (figs. 14 — 21) assumed the essential form of the adult, the division 

 of the body into segments is complete, the antennse have appeared, 

 the mouth with its lip, mandibles, maxillse and accessory append- 

 ages, have acquired nearly their mature condition, and four pairs 

 of true feet are present, the head is quite distinct from the tho- 

 rax, which presents four distinct rings, and the abdomen exhibits 

 five rings and the terminal appendages. The internal ovaries 

 are developed, and the external receptacles are present, but as yet 

 distinct, showing no tendency to coalescence except at their 

 origin. The true feet have assumed a lamellar condition, the 

 rami however are but slightly developed, and the pencil of long 

 bristles with which the internal ramus is furnished suggest to 

 us the feet of certain annelides. As the intercoxal plates have 

 begun to develope themselves, the legs of opposite sides must 

 now act simultaneously. The habits of the animal are remark- 

 ably sluggish, and all its motions, as well indeed as its general 

 physiognomy, remind us strongly of an annelidan. 



In the next stage (figs. 12, 13), the progress of consolidation 

 has become manifest, thus presenting us in this respect with a 

 retrocession towards the early condition of the larva. The head 

 continues distinct from the thorax, but the fourth thoracic ring 

 is confounded with the posterior region of the body. The ovi- 

 gerous receptacles have now become united along the mesial 

 line, and are loaded with ova. The motions are still sluggish. 



In the final stage (figs. 1 — 11) the progress of consolidation 

 has still further advanced, the head has become inseparably 

 united with the first thoracic ring, so that but two segments are 

 now distinct in the thorax. The creature is natatory, and emi- 

 nently active. 



The high development of the reproductive system in the second 

 and third of the phases just described may appear opposed to 

 the opinion here expressed, that these are animals in an imma- 

 ture condition. The objection however will lose all its validity 

 when we recollect that innumerable recent observations go to 

 prove that the exercise of the generative function is by no means 

 necessarily confined to the adult state. Whether however we do 

 or do not admit the specific identity of all the forms now de- 

 scribed, it is certain that they present us with a series of phases 

 in progressive development ; and the light thus thrown upon the 

 morphology of Notodelphys ascidicola is almost entirely the same, 

 whether we view them as different ages of a single species, or as 

 permanent or transitory conditions of several species. 



General Considerations. 

 As to the exact zoological position of our little crustacean, it 



