THE GROWTH OF THE LARVAL SKELETON. 79 



the summit of the stalk, while the orals are the upper ones, receiving 

 their name from their encircling of the mouth. All these plates can 

 be made out in the last stage of the swimming embryo (fig. 1) and 

 characterize the stage of most of the fixed larvaB in the microscopic 

 preparations accompanying this article. A few however show a 

 further stage, where a third row of tiny plates is intercalated between 

 the two original rows of basals and orals; these small plates- are the 

 first radials (r), each is alternate with the larger plates of the 

 skeletal basin. The several rows may be formulated thus : 



o o o o o 



R R R R R 

 B B B B B 



Each ring will be noticed to comprise five plates, the fundamental 

 echinoderm index. The mouth, as before mentioned, is centrally 

 in the calyx formed by these perforate or cribriform plates, and 

 is surrounded by a row of tentacles armed with a limited number of 

 delicate thread-like processes. 



Growth after this is rapid, a second circle of radials appears 

 superposed to the first and then a third upon the second. From the 

 third proceed the arms double the index number, two being borne on 

 each third radial, which has two fascets for this purpose on its upper 

 surface. At the same time the topmost joint of the stalk has been 

 enlarging and becomes a great plate-like structure, the centro-dorsal 

 plate from which arise a number of claw-like jointed organs, the cirri. 



Soon after this, the body with its now long arms, breaks off from 

 its stalk at a point just below the centro-dorsal plate, and enters 

 upon adult life, free at will either to creep amid the mud or 

 rocks, or to swim with rythmic beats of its feather-like arms through 

 the water. It is however doubtful if it makes much use of its 

 powers. It certainly docs not travel far from certain favorite 

 localities, where it is usually found gripping stones or weed with the 

 circle of hooked cirri borne on the centro-dorsal plate. When 

 disturbed, its mode of swimming is extremely graceful, the arms being 

 alternately contracted and expanded as in the pulsations of a medusa. 



For long, the stalked larva w r as considered a distinct animal 

 from the adult, receiving the name Pcutacrimts ctnoprens, as it was 

 believed to be a tiny relative of that large and lovely stalked crinoid, 

 Pcntacrinus caput-mcdnsm, from the Antilles, then known from rare 

 specimens held precious by a few fortunate museums. 



Special interest attaches to this beautiful creature from the great 

 part played by its relations, if not its ancestors, that lived during former 

 periods of the world's history, for the Encrinites whose remains have 



