ECHINODERMA GENERAL DESCRIPTION 



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FIG. IV. 



Larva of Antedon 

 (after Bury). Anterior 

 end, with preoral 

 lobe (PL), uppermost. 

 X 95. 



there is formed a preoral ring which separates from the rest. 



The ventral depression runs up on either side the preoral area, 



and eventually surrounds it and its ciliated ring. In the region 

 of the ciliated rings the body stretches out pro- 

 cesses, which are symmetrically paired, except the 

 frontal process, which bears the preoral ring. In 

 some species this process splits into three branches 

 and the cilia disappear ; such a form is called 

 Brachiolaria. 



A still further development is the Pluteus 

 larva (Fig. II. 1, and Fig. V.) of Ophiuroids 

 and Echinoids, characterised by the decrease of 

 the preoral area and the increase of the anal area ; 

 paired processes extend forward, and an unpaired 

 process stretches backward from the posterior end 

 of the anal area ; these processes are usually very 

 long and supported by spicules, but movable. 

 Two, arising from the posterior and lateral region 



of the ciliated ring, are pronounced in Ophiuroids but absent in 



Echinoids. 



The development of the larval form 



from the ovum is effected in much the 



same way in all known Echinoderma 



(Fig. VI.). The segmentation of the 



ovum is total and quite or almost equal 



(Fig. VI. 2). A coeloblastula is formed 



with a segmentation cavity, and with a 



wall of a single cell -layer, thicker in 



one region (Fig. VI. 3). This region 



is invaginated, forming the archenteron 



of a gastrula (Fig. VI. 4). At about 



the same time the thickened region, 



now the end wall of the archenteron, 



proliferates endoderm cells, some of 



which wander into the segmentation 



cavity, where they may be joined by a 



lesser number from other parts of the 



gastrula wall (ectoderm), and so form mesenchyme, from which 



mesoderm tissue is ultimately developed (Fig. VI. 5). The 



archenteron occupies but a small part of the segmentation cavity, 



its lumen is usually narrow, and the external opening forms a 



small blastopore, which in Antedon soon closes. Both endoderm 



and ectoderm are usually ciliated from the beginning ; but in 



Antedon cilia appear only on the ectoderm after gastrulation. 



The larva becomes bilaterally symmetrical by dorso-ventral 



compression and the formation of a ventral concavity. The inner, 



-o 



Fio. V. 



Pluteiis of a Heart-urchin, from 

 ventral side (after Lang). 0, mouth ; 

 As, anus. 



