THE PLAN OF THE ECHINODERMS. 53 



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

 mental organs ; and if, along with these changes, the aliment- 

 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 Ascidian, the 

 passage is indicated, if not effected, by JBalanoglossus, which, 

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

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

 pharynx of the Tunicata and the circle of tentacula around 

 the oral aperture, with the single ganglion, approximate them 

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

 apertures, which communicate with the exterior, either di- 

 rectly or by the intermediation of an atrial cavity, the Tuni- 

 cata resemble only Balanoglossus and the Vertebrata. The 

 axial skeleton of the caudal appendage has no parallel except 

 in the vertebrate notochord. In the structure of the heart 

 and the regular reversal of the direction of its contractions, 

 the Tunicata stand alone. The general presence of a test 

 solidified by cellulose is a marked peculiarity, but in esti- 

 mating its apparent singularity the existence of cellulose as 

 a constituent of chitin must be remembered. Finally, the 

 tadpole-like larvae of many Ascidian s are comparable only to 

 the Cercariw of Trematodes, on the one hand, and to ver- 

 tebrate larval forms on the other. 



Yet another apparently very distinct type is met with in 

 the extensive group of the Eehinodermata. 



In all the other Metazoa, except the Porifera and Ccelen- 

 terata, the plan of the body is, obviously, bilaterally sym- 

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

 tical plane being similar. Any disturbance of this symmetry, 

 such as is found in some Arthropoda and in many jfollusca, 

 arises from the predominant development of one half. But, 

 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 case in the Actinia 

 or Ctenophoran, this radial symmetry is never perfect, 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 



