193 



Fig. 13. 



of this part has been lost, and fresh locomotor organs are acquired, 

 whilst the ingestive function of the stomach may be readily observed 

 in the living Cydippe. 



The ontogeny ' of this form shews that the stomach repeats its 

 ancestral history, and is invaginated from the epiblast. Above these 

 forms we nearly always find an area of 

 the epiblast invaginated for the function 

 of polycytic ingestion, well-known to 

 Morphologists as the stomodàeum. 



NB. Though quite a speculation — if we ac- 

 cept the homology of the Vertebrate blastopore (at 

 any rate, in part) with the Invertebrate mouth, then 

 the neural tube, in development and structure, 

 answers to a hypertrophied stomodaeum or in- 

 gestive organ, a specialised part of the ecto- 

 dermal locomotor system. As the Inverte- Fig. 13. Diagram of Cteno- 

 brate oesophageal nerve ring takes origin in the phora, showing ingestive (dotted) 

 stomodaeal ingestive area, so from the walls of area, and fresh form of locomotion, 

 the Vertebrate hypertrophied ingestive tract (pri- not indicated, 

 mitive groove) the elongated dorsal nerve area 



(an elongated ring forming a cylinder) arises and persists after the ingestive func- 

 tion aborts. This function probably persists in the typical Chordate larva. 



We can thus trace the gradual differentiation of both the mono- 

 cytic and the polycytic ingestive organs, as exemplified by parallel 

 series taken respectively from the Protozoa and the Metazoa, but these 

 monocytic and polycytic elaborations of organs are quite independent 

 of one another, and we have to follow up the function of monocytic 

 ingestion in the Metazoa, and if possible attempt to elucidate the 

 inception of polycytic ingestion therefrom. 



In the transition from the Protozoa to the Metazoa the underlying 

 principle is acknowledged to be the subordination of the monocytic 

 individuality, and differentiation to that of the Polycytic, or in other 

 words, the inception of the individuality of the unit of the higher 

 order at the expense of that of the lower order ^. Thus we shall expect 

 to find in the ingestive process a cessation of further adaptations to 

 favour monocytic ingestion in so far as it benefits the single cell, and 

 the evolution of further processes by which the activity of the ingestive 

 cell may benefit not only itself, but its surrounding cells. 



Thus, if we take a spherical multicellular colony (Fig. 1) in which 

 each cell has locomotor organs, either cilia or flagella. Supposing 

 this colony to evolve upon the lines of monocytic differentiation, each 



^ A. Agassiz, Embryo of the Ctenophorae. 

 — F. M. Balfour, Comp. Embr.^Vol. I. 



8 H. Spencer, Principles of Biology. 



Mem. Amer. Acad. Arts and Sc. 



11 



