136 PRIMITIVE FORMS OF LIFE 



are thus constantly being forced outwards, and five arms are 

 formed behind them, the ventral surface following step by 

 step the growth of the dorsal surface. Among the Sea-urchins 

 new plates are not formed between the dorsal ones, which 

 remain united around the anus, but the ventral surface under- 

 goes a rapid growth, so that the animal swells like a soap- 

 bubble suspended from the pipe by which it is blown. The 

 Holothurians are scarcely more than Sea-urchins whose 

 skeleton has become reduced to spicules. Among the Crinoids 

 the primitive plates remain united as in the case of the Sea- 

 urchins, but between the centro-dorsal and the basal plates 

 there is formed a layer of plates in the shape of a long peduncle, 

 by means of which the animal fixes itself. The ventral face 

 is not developed, but outside the radials, which remain united 

 to the basals, under the stimulus of the genital organs there 

 occurs an active budding which gives rise to five arms that 

 may remain simple (Hyocrinus, Rhizocrinus, Democrinus, 

 Eudiocrinus), may bifurcate (Antedon), or ramify in various 

 ways. Finally, the Cystids are fixed like Crinoids, and it appears 

 that, contrary to what occurs in Sea-urchins, only their dorsal 

 surface is developed. We may once again enunciate the 

 proposition : all that it is possible to achieve is achieved. 



The phylum of the Mollusca has developed by analogous 

 changes of posture. All zoologists have been struck by their 

 resemblances to the Worms, either at birth, when they take on a 

 form very like the initial form of these last, or in various organic 

 characters of the adult state. One class only, the Amphineura 

 or Chitonidae, show actual segmentation of the bod}". Two 

 others, the Cephalopoda and the Gastropoda, are characterized 

 by the transformation of their dorsal surface into a large 

 cone, which must have grown in opposition to gravity if 

 the creature's posture had always been what it is to-day. This 

 cone does not exist among the Lamellibranchiata ; but, as we 

 shall soon see, it remains characteristic of the Molluscs. It was 

 straight, among most of the older Molluscs, Cephalopods 

 (Orthoceras) and Gasteropods (species of Tentaculites, 

 Conidaria, Hyolites, etc.) ; it has persisted in most of the 

 present Cephalopods (squids and octopuses) ; it is rolled up in a 

 plane spiral so as to retain the primitive symmetry in the 

 majority of the shelled Cephalopods (Nautilidse, Goniatitidae, 



