Chap. II. MORPHOLOGY AND NOMENCLATURE. 79 



of an Echinus and a common Star-fish, as consisting of five converging spheromeres, 

 as the Ijody of a Caterpillar or a Buttertly consists of thirteen rings movable 

 upon a longitudinal axis. In most Crinoids we have also five spheromeres, but 

 occasionally four or six, and in some Asterioids even a larger number. In Aca- 

 lephs the body is generally built of four or eight or twelve spheromeres ; but here 

 and there the numbers vary, as we find also that the number of rings varies in 

 the lower Worms. In the Halcyonoid Polyps the number of spheromeres is 

 constantly eight, they being the highest Pol}^^"^. In the Actinoids we find, in the 

 lowest families, a large and varying number of spheromeres, sometimes increasing 

 regularly with age ; whilst in the highest Actinoids — the Madrepores proper — the 

 individual Polyps are made up of twelve spheromeres, six of which are more 

 2)rominently developed than the six others. A similar difference between alternating 

 spheromeres is observable among the higher Acalephs. Here unequal spheromeres 

 may coml^ine in such a manner as to produce the appearance of bilateral symmetry ; 

 and though this feature is not only common among Radiates, but even prominent 

 in some of the higher representatives in each class of this type, it is yet subordi- 

 nate to the plan of their structure : for, upon close analysis, it is found, that, even 

 in those Radiates in which bilateral symmetry is most marked, it is in i-eality 

 the result of a symmetrical arrangement of radiatmg elements around a vertical 

 axis, and not of elements symmetrically placed upon the two sides of a longi- 

 tudinal axis. 



Thus it appears that the body of all Radiates, be they Polyps, Acalephs, or 

 Echinoderms, is composed of identical elements, which may be called spheromeres ; 

 and that these parts are arranged symmetrically around a vertical axis, in the 

 same manner as the wedge-shaped segments of an orange are arranged Avithin its 

 bark. There is no pi-opriety, therefore, in considering the body of Acalephs as 

 something peculiar, and different from that of a Polyp or an Echinoderm, and it 

 is unnecessary to give it a distinct name, as Huxley does, who calls it Hydrosoma, 

 else this name must be extended to all Radiates ; for the body of the Actinia is 

 as much a Hydrosoma as that of any Acaleph, and so also is that of Pluteus and 

 allied forms (young Ophiurioids and Echinoids), that of Bipinnaria and Brachiolaria 

 (young Asterioids), and that of Auricularia (young Holothuria). We need, however, 

 distinct names to designate the different stages of development of these animals; 

 which, once sanctioned by use, may become as significant as the names applied 

 to the larval conditions of the Insects. 



I should not object to the name of Hydrosoma for the young Acalephs, had we 

 not already, for every stage of their growth, names which are very generally adopted, 

 and which render new ones superfluous. For the earliest state of the embryo 

 Hydroids we have the name of Planula, for the Medusas buds of the Hydroids that 



