256 POLYZOA INFUNDIBULATA. 



connexion is effected by means of an inner tunic, which, after 

 inclosing the polype's body as in a pouch, is afterwards re- 

 flected over the aperture of the cell, the reflected portion 

 becoming exterior and solidified either by calcareous deposi- 

 tions in its texture, or by a mutation of its thin membranous 

 character into a horny investment better suited to the office it 

 has now to perform of protecting the sentient body from a too 

 rough contact of the medium in which the animals live, and 

 from worse foes. From this mode of connexion it results 

 that when the polypes retire within, they at the same time 

 must close the aperture to their cells, for that portion of the 

 inner tunic which is pushed outwards by their exit, in their 

 withdrawal follows the body by a process of invagination, be- 

 coming at one and the same time a sheath for the column 

 of tentacula, and a plug to the aperture, which, when of a 

 flexible material, has its margin also drawn tighter and closer 

 together. 



The polype which endues itself with this cell is widely 

 different from any we have yet described. Between the 

 polypidoms, however, there is often an apparent affinity. 

 The Crisiadse are not unlike the Sertulariadse, and it is still 

 disputed whether some Gemmicellarise appertain to this family 

 or to the Flustrse : the resemblance between the Sertularise 

 and the Vesiculariadse misled even Lamarck to their union 

 under one genus ; and their names would seem to imply that 

 the framers of the genera Alcyonidium and Alcyonella be- 

 lieved them to be. in a family relationship to Alcyonium. 

 These are examples which prove the fallacy of outward cha- 

 racters ; and how darkly the naturalist must grope his way 

 who would walk through Nature's labyrinth without the 

 Ariadnacan thread that the anatomist alone can give him ! 

 In the instance before us he has demonstrated that the 

 resemblances indicated above imply no propinquity in their 

 objects. The Polyzoa or ascidian polypes the Creator has 

 cast in the mould not of the Radiata, but of the Mollusca, 

 yet with such a considerable variation as to mark their or- 

 dinal distinctness ; for the Mollusca tunicata, their nearest 

 allies, are not protrusive from their cells as these polypes 

 are ; and this seeming slight discrepancy connects itself, per- 

 haps of necessity, with a total change in the position and 



