520 ' MR J. D. MACDONALD ON THE MORPHOLOGICAL RELATIONSHIPS 



theoretical passage of the type upon which it is constructed into higher and 

 lower types is effected in the tentacular system by addition, and in the alimen- 

 tary system by modification, and these are, of course, either progressive or retro- 

 gressive, as they tend to the higher or lower forms. Thus, the tentacula of the 

 Poly soon, and the branchial tentacula of the Ascidian^ are progressive additions, 

 while the oral tentacula (so-called) of the Actinozoa, and then the oral tentacula 

 of the Hydrozoa, are retrogressive additions, however contradictory the phrase 

 may appear.* On the other hand, the insulation and simple flexure of the in- 

 testine in Polyzoa, and the perfect looping or double flexure of the intestine in 

 Tunicata, are progressive modifications, while the simplification and central en- 

 largement of the somatic cavity, abolishing the intestine in Actinozoa^ and, finally, 

 the virtual eversion of the stomach in Ilydrozoa, are retrogressive modifications. 

 To make the principles here expressed more intelligible, I have arranged them in 

 a tabular form, thus, — 



Conversions of the Cydippean Type. 

 I. 



MODIFICATIONS AFFECTING THE ALIMENTARY SYSTEM. 



* Progressive, 



Complete insulation of the intestine from the somatic cavity. 



a. Primary type, with simple neural flexure — Brachiopoda and Polyzoa. 



b. Secondary type, with primary h^m^il and final neural flexure — Ascidiozoa. 



** Retrogressive, 

 a. Primary type. Stomach more completely insulated by the abolition of the intestine — 



Actinozoa. 

 b. Secondary type. Eversion and consequent abolition of the true stomach — Hydrozoa. 



11. • ,^ 



ADDITIONS AFFECTING THE TENTACULAR SYSTEM. 



* Progressive. 



a. Primary type. Oral tentacula of Brachiopoda and Polyzoa. 



b. Secondary type. Branchial tentacula of -4scio?to2oa. 



** Retrogressive. 



a. Primary type. Oral tentacula* of ^cJmoroa. 



b. Secondary type. Oral tentacula of Hydrozoa. 



Two suggestions present themselves in this mode of considering the subject,— 

 viz., first, that the Bradiiopoda and Polyzoa should be taken together as a group 

 in themselves, quite as natural as the Tunicata, consisting of simple and com- 

 pound forms ; and, secondly, that the Ctenopliora should be received within the 

 pale of the Molluscoida, perhaps as a pelagic section representing Salpa, Doliolum, 



* It will be observed that the homologues of the oral tentacula of Tuhularia do not exist in 

 Lucernaria, while they are the sole prehensile organs of Coryne and Hydra. 



