EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 403 



consists in (l) MacBride's description of abnormal larvae in which a starred hydrocoele 

 was present on each side. (2) My own discovery of Cribrella larvae like fig. 95, in 

 which the right and left sides have exchanged form and function, resulting in an 

 enantiomorph of the normal. Such enantiomorphs could not be recognised as such 

 after the pre-oral lobe had been lost, and may therefore, like enantiomorphic flounders, 

 be fairly common in nature. (3) Inferences from such types as Actinotrocha and 

 Cephalodiscus. 



However, it seems safer, with present evidence, to suppose that the bilateral ancestor 

 was sufficiently provided with ciliated tracts for feeding purposes, and that the 

 tentacles were of later origin. Unquestionably the nervous and vascular systems were 

 in evidence, but for the present we may leave these out of consideration. 



We owe the identification of the anterior ccelom and its relationship to the axial 

 sinus and water-pore to Bury, and it is therefore peculiar that he should, in constructing 

 his prototype, have assumed a primitive bilateral condition for this organ ; he also 

 supposed that the left very early became reduced to a mere remnant, and that the right 

 had disappeared altogether. There does not appear to be any satisfactory evidence for 

 supposing that this coelom was ever paired. 



Again, to Bury we owe the discovery of the ' dorsal sac,' a median pulsating sac 

 over the oesophagus, subsequently observed by Field, and stated by him to arise as a 

 schizocoele. There can be little question that the central ccelom here described is the 

 same organ.* 



This bilateral ancestor can be directly compared to Balanoglossus. The anterior 

 coelom (its pre-oral part) has already been compared by Bury to the proboscis cavity 

 of Balanoglossus, the water-pore to a left proboscis pore, and the central coelom (dorsal 

 sac) to the pericardium (herzblase). These homologies I adopt from him, with the 

 reservation with regard to the unpaired nature of the anterior ccelom. Beyond this 

 I cannot go with Bury. He makes a special attack against the assumption that a fixed 

 stage is necessary to account for the axial symmetry of echinoderms. I have elsewhere t 

 attempted to gather together the various laws regulating the symmetry of animals, 

 and have tried to show that they depend on the inter-action between an organism and 

 its environment along definite lines. Such being the case, it seems that an 'ancestor' 

 must be constructed, not only having ontogenetic changes in mind, but also taking into 

 consideration the probable surroundings of the animal at each period. For example, 

 Bury proposes to show that a fixed stage is not a necessary assumption, and proceeds 

 to construct an ideal ancestor (bilateral) in which the oesophagus is already formed 

 into a closed ring with five tentacles. This ancestor, as he himself points out, closely 

 resembles the 'Pentactaea' conception of Semon. In each case the whole question of 

 the hydrocoele ring is avoided by starting with this assumption. The 'ancestor' of Bury 



* This will be confirmed later by a description of its later changes. In the adult it has the relationship of a 

 heart-vesicle or pericardial vesicle to the blood-vascular system, the presence of which, I hope, as indicated elsewhere, 

 to show in detail later {Proc. Royal Physical Soc, March 1901). 



t Masterman, A. T., Natural Science, January 1899. 



