MACLEAY. 



603 



lowest in their sub-kingdom. Lamarck and Milne -Edwards 

 go so far, indeed, as to maintain, that they have a nearer 

 relationship to the Polypes than to the Mollusca ; and 

 we may safely consider them to be an "osculent" group 

 with Macleay, connecting the Mollusca with the Ascidian 

 Zoophytes. To use Mr. Maclcay's words,—" The Tuni- 

 cata, then, are animals which connect the Acrita, or lowest 

 primary division of the animal kingdom with the Mollusca. 

 From the Mollusca, however, they differ in having an exter- 

 nal covering, consisting of an envelope distinctly organised 

 and provided with two apertures, of which one is branchial, 

 the other anal. They also differ from the Mollusca as well 

 in their mantle forming an internal tunic corresponding to 

 the outer covering or test, and provided, like it, with two 

 openings, as in ha\-ing branchiae which occupy all, or at least 

 part of the membranaceous cavity formed by the internal sides 

 of the mantle. From the Acrita they differ in_ having dis- 

 tinct nervous and generative systems, while their intestinal 

 canal is provided with two openings, both internal."* 



This osculent group Mr. Macleay divides as follows, in a 

 rather futile attempt to make the arrangement square with 

 his system of quinary circles : — 



TUNICATA. 



Aberrant Group ? 



I. Tethya. 



Or such Tunica- 

 ta as have their 

 mantle adhering to 

 the envelope or test 

 only at their ori- 

 fices ; — their bran- 

 chiae regular, con- 

 stituting the sides 

 of the respiratory 

 cavity ; and their 

 branchial orifice sur. 

 rounded by a mem- 

 branaceous ri'ig, 

 which in general 

 is supplied with 

 tentacula, as in Po- 

 lypes. 



Families. 



1. AsciDiD-s;. 

 Tethi/es simples, 

 Sav. 



2. BOTRYLLID^. 



Teth. composees, 

 Sav. 



3. LUCID.E. 



Lucies, Sav. 



Animals. 



Generic Types. 



Simple and fixed, 

 having their 

 orifices exter- 

 nally irregular. Ascidia. 



Compound and 

 fixed, having 

 their orifices 

 externally re- 

 gular PoLYCLINUM. 



Compound and 

 floating having 

 their branchial 

 cavity open at 

 the two extre- 

 mities Pyrosoma. 



* Linneean Transactions (1824) xiv. 631. 



