70 ACALEPIIS IN GENERAL. Part I. 



arranged radiatingly l:)<.'t\veeii the aniltulacral rows, with which they alternate. This 

 arrangement is strict!}' homologous to that of the sexual organs of the Polyps and 

 Acalephs ; for in Polyps the oyai-ies and spermaries hang from the edges of the 

 radiating partitions, and in Acalephs they are placed upon the sides of the radi- 

 ating tubes, or, what amounts to the same, they alternate with the radiating cham- 

 bers in Polyps and with the radiating tubes in Acalephs, as they alternate with 

 the amljulacral system in Echinoderms. That in Echinoderms the ambulacral 

 system is more or less complicated, assuming now the appearance of gill-like ten- 

 tacles around the oral aperture in Holothuria) and Echini, and now that of simple 

 tuljes with external suckers, as in most memljers of this class, does in no way 

 alter the primary organic relations of these parts. The homological identity of the 

 ambvdacra of the Echinoderms with the radiating tubes of the Acalephs is most 

 easily ascertained b\- comparing that system in those Ilolothuria^ which have no 

 external ambulacral apjiendages with the disposition of the radiating tubes in the 

 Ctenophora). In Synapta, for instance, and in allied genera, the ambulacral s^'stem 

 consists of tubes as sim23le as the radiating tubes of the naked-eyed Medusae ; 

 while in some Beroid Medusae, such as Bolina, Alcinoe, and Mnemia, the radiating 

 tubes are really more complicated than the amljulacral tuljes of Sj-najita. This 

 apparatus is so strictly homologous in both families, that the Cteno2)horai may 

 fairly I)e said to possess an ambulacral system identical in its general disposition 

 with that of the lower Holothuria?. Even the form of some of the Ctenophora^, 

 such as Beroe proper, Idyia, etc., recalls that of the IIolothuria\ 



At the perijjheric ends of the ambulacral system of a large number of Echi- 

 noderms there are ocelli, which deserve a special notice in this connection. Aljove 

 each of these ocelli there is frequently an odd amindacral tube, particularly promi- 

 nent in some Star-fishes. This odd ambulacral tulie bears the same relation to 

 its ocellus as the hollow tentacle of a Sarsia bears to the ocellus at its base ; 

 and l)otli have an homologous connection with their respective aquiferous systems. 

 In Sarsia, each hollow tentacle with its ocellus communicates in the same manner 

 with the corresponding radiating tulje, as the odd ambulacral tul)e of a Star-fish 

 with the whole ambulacral system of its ray. When I represent the andjulacral 

 system of the Echinoderms as homologous to the radiating tubes of the Acalephs 

 and to the radiating chamljcrs of the Polyps, I do not overlook the diHerence 

 there is between them in structure and in functions. But these differences consist 

 in a more or less complicated structure and more or less specialized functions, 

 as we frequently ol)serve even l>etwcen members of one and the same class and 

 not in a tj'pical modification. Similar differences exist among Echinoderms taken 

 as a class, and even among different fauulies of the same order of that class. In 

 some Star-fishes the digestive cavity is a blind sac, while in others it is open at 



