Common Crayfish. lis 



and which has a high degree of independence manifested by its nerve 

 ganglia in Podophthalmata and Hedriophthalmata. 



The number of the prae-oral segments being thus taken as three, and 

 the so-called ' lips' of certain Crustacea being eliminated from the enu- 

 meration as not rejiresenting appendages, but being merely indurations 

 of the lining membrane of the digestive tract homologous with the 'jaws' 

 of the leech, we obtain, by omitting, for the reasons above stated, to 

 count the telson as a segment, seventeen as the typical number of post- 

 oral, and twenty as the typical number of the entire series of segments 

 in Arthropoda. The appended Table shows how these views may be 

 applied to the Insecta, the Arthrogastrous Arachnida, Myriapoda, and 

 to the two above-named orders of Crustacea, as also to the Copepoda ; 

 which, in spite of their small size, with which inferiority of organization 

 has been supposed to be commonly correlated in Crustacea, present many 

 important points of affinity to the highest order in the class. Amongst 

 these may be mentioned the degree to which heteronomy or diffei'entia- 

 tion is can-ied out in the various regions of the body ; and the external 

 similarity which these small animals thus obtain to such Crustacea as 

 Peneus and Mysis is made the more striking, when we recollect that 

 they all alike leave the egg with no other appendages than those of the 

 ' Nauplius ; ' and that the adult Copepod corresponds very closely, if not 

 exactly, as to the number of articulated appendages on its ' fore-' and 

 'middle-body,' with certain 'Zoea' stages in the development of Podoph- 

 thalmata. See Spence Bate, Phil. Trans. 1858, pi. xl. figs. A and B; 

 Glaus Die frei Lebenden Copepoden, 1863, Taf. xxxiii. fig. 6; Dias 

 Longiremis, Taf, xix., fig. 2, Thalestris harj)actoides^. The non-seg- 

 mentation of the post-abdomen in many of the parasitic Copepoda, as 

 also in the Cladocera, Daphnis, the Ostracoda, Cypris and the Cirripedia, 

 eliminates them as it does the non-Arthi"ogastrous Arachnida from con- 

 sideration, except as to the anterior regions of the body. The Myriapoda 

 together with a few Branchiopoda, Apus, and the extinct Trilobites, are 

 similarly eliminated for the opposite reason. 



The term ' Hedriophthalmata,' Schicidte, Ann. and Mag. Nat. Hist., 

 Ser. iv., vol. i., 1868, p. 6, is employed in the following Table in the 

 same restricted sense as its etymological equivalent, by Professor West- 

 wood and Mr. Spcuce Bate, iu their work on the Sessile-eyed Crustacea. 



* The internal anatomy of the Copepoda is well ilhiatrated in the former of these 

 two figuren ; the dorsal opening of the anus being especially noteworthy, aa corre- 

 sponding with a condition observed by Rathke, in the early stages of the develop- 

 ment of the Astacus, see p. 97, supra. 



I 2 



