154 Dr. \Y. T. Caiman on the 



With regard to these and other points of internal anatomy 

 our knowledge is very incomplete for many of the groups. 

 Nothing is known of the internal anatomy of the Lopho- 

 gastrida?, and very little regarding the Euphausiidse and 

 the lower decapods. One point which seems to tell against 

 the system of classification here advocated may be given for 

 what it is worth. This is the presence in all of the Podo- 

 phthalmate groups [Anaspides ?, Mysidce, Euphausitdce, 

 Decapoda, Stomatopoda) of an unpaired descending artery 

 originating from the posterior end of the heart or from the 

 base of the posterior aorta (superior abdominal artery) and 

 perforating the nerve-cord to become connected with the sub- 

 neural artery (eternal and inferior abdominal arteries). In 

 the Edriophthalinate orders no similar arrangement is known, 

 the subneural artery, where it exists, being connected with 

 the dorsal poition of the vascular system by paired lateral 

 arteries or by a circumcesophageal ring. In view of the 

 great divergences which may exist in the disposition of the 

 arterial trunks within the limits of a single order (e. g. the 

 Isopoda), no great taxonomic importance can at present be 

 attached to such differences. 



Besides the characters, summarized in the definitions given 

 below, which hold good throughout the various orders brought 

 together in this classification, there are many connecting 

 characters which serve to link together the individual orders 

 and to indicate their affinities, although they cannot con- 

 veniently be included in our definitions. Many of these are 

 discussed in the papers of Boas and Hansen, and we may 

 simply mention as examples the retroverted palp of the 

 maxillula in Lophogastridae (Mysidacea), Cumacea, and 

 Tanaidacea, the branchial epipod of the first thoracic appen- 

 dage in the same orders, and the distinct, though immovable, 

 ocular peduncles of the Tanaidacea. On the other side the 

 Euphausiacea share with some suborders of the Decapoda 

 the possession of an appendix interna on the pleopods, and 

 the elaborate copulatory armature of the first pair of pleopods 

 in the former group recalls that of the Penasidea in the latter, 

 although differing in details. The larval development of 

 the Euphausiacea runs closely parallel to that of the Penasidea, 

 and Dr. Hansen's recent discovery * in a species of Sergestes 

 of luminous organs resembling, though of somewhat different 

 structure from, those of the Euphausiacea, helps still further 

 to diminish the narrow space which separates the two. 



* Proc. Zool. Soc. London, 1903, i. p. 72. 



