Primitive Cell-layers of the Emhryo. 335 



containing, as in the case of Annelids, haemoglobin. The 

 exceptional development of such a subdivision of the blood- 

 lymph-space, unparalleled throughout the whole group of Ar- 

 thropoda, is additional evidence in favour of the view that the 

 primitive blood-ljmph-space readily lends itself to the develop- 

 ment of variously distributed and communicating vascular 

 systems, even systems as special as the ambulacral and respi- 

 ratory systems of Echinoderms. 



The relation of the segment-organs to the primitive blood- 

 space has already been spoken of. There is considerable 

 ground for regarding it as constant throughout the Triploblas- 

 tica, as the blood-lymph-space itself is constant. It appears 

 under various modifications as a canal, often ciliated and 

 funnel-like, forming a communication between part of the 

 blood-lymph-space and the exterior — as, for example, the brown 

 tubes and the cloacal tree of Gephyrea, the organ of Bojanus, 

 the Fallopian tubes and seminal ducts of sharks, and more 

 doubtfully in Echinodermata and Arthropods. 



The Triploblastica not only exhibit this unity of type as 

 regards their chief viscera, but there are certain regions of the 

 body which must be considered identical in all ; especially 

 must the prostomium or region in front of the mouth, the axis 

 of anterior growth, where it is persistent, be held to be homo- 

 genous throughout the series. It is in relation with this 

 " head-flap " that the primitive nerve-centres are developed 

 and always make their appearance as the great sensorial gan- 

 glion-masses. Already in the free-swimming larv^ of some 

 Diploblastica, such as Actinia^ the prostomium is indicated, 

 having a necessary mechanical relation to bilateral symmetry 

 when the mouth is placed anteriorly and locomotion is 

 parallel with the alimentary axis, though here we must 

 not overlook the distinct character of the Diploblastic and 

 Triploblastic mouths. The large primitive tentacle of the 

 young Actinia is a prostomium, and only loses its superior 

 overhanging character as regards the mouth when the animal, 

 abandoning locomotive habits, fixes itself and develops other 

 processes around the mouth which soon equal the first in size. 

 The prostomium in Triploblastica is liable to be suppressed alto- 

 gether in the course of individual development, the mouth 

 becoming terminal or other modifications arising ; but where it 

 does appear it constantly carries the chief organ of sight, 

 whilst the auditory sac is prostomial in Turbcllarians, but 

 metastomial in Tunicates, Vertebrates, and Mollusca. 



The production of individuals of an increased complexity of 

 organization among Triploblastica, by the linear aggregation 

 of zooids, produced by budding in the posterior or metastomial 



