THE PLAN OF THE ECIIINODBRM8. :,{ 



appears to represent the water-vascular system and the seg- 

 mental organs ; and if, along with these changes, the alii: 

 ary, circulatory, respiratory, genital, and sensory organs take 

 on special characters, we arrive at the complete Molluscan 

 plan. 



From the Turbellarian to the Tunicate, or Arcidian, the 

 passage is indicated, if not effected, by Balanoy i h, 



in its larval state, is comparable to an AppendictUuria with- 

 out its caudal appendage. On the other hand, th. large 

 pharynx of the Tunica ta and the circle of t nta. ula around 

 the oral aperture, with the single ganglion, aj.pi 

 to the Polyzoa. In the perforation of the pharynx by lateral 

 apertures, which communicate with the exterior, eith -r 

 rectly or by the intermediation of an atrial n.vity, t! / 

 cata resemble only Balanoglo&sus and the Vertebrata. The 

 axial skeleton of the caudal appendage has no pan-ill- 

 in the vertebrate notochord. In the structure of tin- 1 

 and the regular reversal of the direction of its contractions, 

 the Tunicata stand alone. The general presence of a 

 solidified by cellulose is a marked peculiarity, hut in 

 mating its apparent singularity the existence of r-liul.se as 

 a constituent of chitin must be remembered. Finally, the 

 tadpole-like larvae of many Ascidians are comparable on! 

 the Cercarice of Trematodes, on the one hand, and to 

 tebrate larval forms on the other. 



Yet another apparently very distinct type is met with in 

 the extensive group of the Echinodernt 



In all the other Metazoa, except the Porifera and Co?/ew- 

 terata, the plan of the body is, obviously, bilaterally IJ 

 metrical, the halves of the body on each side of a median 

 tical plane being similar. Any disturbance of this sv 

 such as is found in some Arthropoda and in in.-mv M<>ll> 

 arises from the predominant development of one half. 1 

 in a Sea-urchin or Starfish, five or more similar sets of parts 

 are disposed around a longitudinal axis, which has the mouth 

 at one end and the anus at the other ; there is a radial sym- 

 metry, as in a sea-anemone or a Ctenophoran. Nevertheless, 

 close observation shows that, as is also the oasc in tho Actinia 

 or Ctenophoran, this radial symmetry is n and 



that the body is really bilaterally symmetrical in relation to 

 a median plane which traverses the centre of length of one 

 of the radiating metameres. 



Another marked peculiarity of the Echinoderm type is 



