132 HISTORY OF CRUSTACEA. CHAP. XII. 



extended position, like the larvae of Achtheres enclosed 

 by the Nauplius-skin. On the other hand the uni- 

 formity of development that prevails in each of the two 

 orders which is expressed in the Amphipoda for exam- 

 ple in the formation of the " micropylar apparatus," in 

 the Isopoda in the want of the last pair of ambulatory 

 feet testifies that the present mode of development has 

 come down from a very early period and extends back 

 beyond the separation of the present families. In these 

 two orders also, as well as in the Crabs, we can hardly 

 hope to find. traces of earlier young states, unless it 

 be in the family of the Tanaidae. 4 If any one will 

 furnish me with an Amphipod or an Isopod with Nau- 

 plius-brood, the existence of which would not be more 

 remarkable in independently produced species than 

 that of a Prawn with Nauplius-brood, I will abandon 

 the whole Darwinian theory. 



With regard to the Crabs, and also to the Isopoda 

 and Amphipoda, we were led to the assumption, that, 

 about the period when these groups started from the 



4 Whether the want of the abdominal feet in the young of Tanais be 

 an inheritance from the time of the primitive Isopoda, or a subsequently 

 acquired peculiarity, which appears to me the more admissible view at 

 present, may perhaps be decided with some certainty, when we become 

 acquainted with the development and mode of life of its family allies, 

 Apseudes and EJma. The latter, as is well known, is the only Isopod 

 which possesses a secondary flagellum on the anterior antennae. I have 

 recently obtained a new and unexpected proof that the Tanaidx 

 (" Asellotes he'teropodes " M.-Edw.) of all known Crustacea approach 

 most closely to the primitive form of the Edriophthalma. Mr. C. Spence 

 Bate writes to me : " Apseudes, as far as I know, is the only Isopod in 

 which the antennal scale so common in the Macrura is present on the 

 lower antennae." 



