278 PHYL UM ECHINODERMA. 



anterior is less distinct), and a posterior terminal tuft. Eventually the 

 posterior end is elongated to form, in the pentacrinoid stage, an attach- 

 ing stalk, which is afterwards absorbed. As all the extinct Crinoids 

 are permanently stalked, there is here an instance of Recapitulation. 



Holothuroids . The larva of Holothuroids (an Auricularid] is much 

 quainter. Its diffuse cilia are succeeded by a wavy longitudinal band, 

 which in the pupa stage breaks into transverse rings, usually five in 

 number. The pre-oral region becomes large. 



Asteroids. Nearest the Auruularia is the larva of starfishes, which 

 has the same enlarged pre-oral region. There are two ciliated bands, 

 of which the ad-oral is smaller, the ad-anal much larger. They are 

 extended peripherally by the development of soft bilateral arms, and 

 such a larva is known as a Bipimaria* But another larval form in 

 Asteroids is the Brachiolaria stage, in which three warty arms are 

 formed at the anterior dorsal end, independently of the ciliated bands. 



Ophiuroids and Echinoids. In the Pluteus larvae (Fig. 131) char- 

 acteristic of these classes the pre-oral region remains small, while the 

 post-anal region becomes large. There is one undulating ciliated 

 band, the course of which is much modified by the growth of six long 

 arms, with temporary calcareous supports. This quaint form is often 

 compared to a six-legged easel. 



The development of these larval forms into the adult is very intricate. 

 The adult is a new formation within the larva, retaining the water- 

 vascular system and mid-gut, but absorbing or rejecting the provisional 

 larval structures. As certain parts are broken down, others are built 

 up, chiefly through the agency of the wandering amoeboid cells of the 

 mesenchyme. The first steps in the upbuilding of the adult, and 

 especially of its skeleton, are to some extent parallel in the five classes. 



One of the most important changes is that from bilateral to radial 

 symmetry. In connection with this, it has been conjectured that the 

 primitive ancestor was bilaterally symmetrical, and that the radiate 

 symmetry was acquired by early sessile or sedentary Echinoderms, such 

 as the Cystoids. As we have already seen, the adults in the different 

 classes tend to acquire an independent and secondary bilateral symmetry. 



It is very difficult to compare the Echinoderm larvae, even in their 

 simplest form, with those of other animals. The nearest type is perhaps 

 the Tornaria of Balanoglossus, but it again is very unique. One 

 naturally tries to compare the Echinoderm larva with the Trochosphere 

 of Annelids, but the differences are very marked. One of the most 

 marked of these is the absence of the apical sense organ, so charac- 

 teristic of the Trochosphere. The fact that this is represented in the 

 larva of Antedon is regarded by many naturalists as a point of much 

 importance. 



RELATIONSHIPS OF ECHINODERMA 



The Echinoderms form an exceedingly well-defined phylum, but 

 the Holothurians especially show how many of the significant char- 

 acters may be lost. In that class we see how the power of forming a 

 calcareous skeleton, the characteristic tube-feet, and the greater part of 



