REPORT ON THE SIPHONOPHOR^E. 9 



physa, Arethusa, &c, in which each cormidium consists of a siphon with a tentacle and 

 of one or more gonophores. 



4. The Athorome of Physophora and of the Anthophysidee, in which each cormidium 

 consists of a siphon with a tentacle, one or more palpons, and one or more gonophores. 



5. The Crystallome of Crystallodes, Anthemodes and other ordinate Physonectse, in 

 which each cormidium consists of a siphon with a tentacle, one or more palpons, one 

 or more gonophores, and a group of bracts. 



Much rarer and much less manifold are the ordinate polygastric corrnidia, in which 

 each group of persons contains several siphons (each with a tentacle) ; such occur in 

 Apolemia among the Physonectse, and in Salacia among the Cystonectse. The line of 

 the stem, in which the cormidia originally bud forth in uniform succession, is the ventral 

 median line of the protosiphon ; usually it is rolled up in a wide or narrow spiral, rarely 

 it remains straight (Crystallodes, Stephanomia). 



STEM or TRUNK. 



(Ccenosome, Ccenosarc, Axial Body.) 



The stem of the Siphonophorse, or the central axial body, on which all the various 

 persons and organs of the corm are attached, is generally compared to the stem of a 

 Hydropolyp stock. This comparison is in our opinion in the accurate sense illegitimate ; 

 for in the latter the primary larval body from which the corm develops is a Polyp-person, 

 while in the former it is a Medusa-person. The comparison holds so far, however, 

 inasmuch as the trunk is branched in both cases. It is not logical to describe the 

 stem of the Siphonophorse as " unb ranched " as is generally done. In reabty it is 

 always branched, for all the appendages — whether they be interpretable as persons or as 

 organs — arise as lateral branches of the axial body. Only the forking or dichotomous 

 ramification is here absent. Further, the conventional description which is generally 

 given of the stem or axial body of the Siphonophoras is strictly applicable only to one 

 legion of the class — to the Siphonanthse. For it is here only that the stem (whether it 

 be long or short) is formed from the primary siphon, and has all its appendages (or 

 branches) budded off in a row from its ventral median line. That they are subsequently 

 often radially disposed depends wholly upon a secondary spiral twisting of the stem, 

 and displacement of its appendages. It is quite different in the second legion, the Discon- 

 anthse. Here the proper stem, i.e., the common central portion of the corm, is formed 

 from the primary umbrella, and all its appendages (or branches) bud out from its lower 

 or subumbrellar surface, not in one row, but in concentric circles or rings, which are 

 originally octoradial. On the other hand the primary siphon in this legion has only the 

 value of a central nutritive organ. 



(ZOOL. CHALL. EXP. — PART LXXV1I. 1888.) Hhhh 2 



