278 PHYLUM 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 Aurécularza) is much 
quainter. Its diffuse cilia are succeeded by a wavy longitudinal band, 
which in the fuga stage breaks into transverse rings, usually five in 
number. ‘The pre-oral region becomes large. 
Asterotds,—Nearest the Auriculariéa is the larva of starfishes, which 
has the same enlarged pre-oral region. There are ¢wo ciliated bands, 
of which the ad-oral is smaller, the ad-ana] much larger. They are 
extended peripherally by the development of soft bilateral arms, and 
such a larva is known as a Szpznnaréa. But another larval form in 
Asteroids is the Avrachzolarta stage, in which three warty arms are 
formed at the anterior dorsal end, independently of the ciliated bands. 
Ophiuroids and Echinotds.—In the Pluteus larve (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 amceboid 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 larva, 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 Axtedon 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 
