390 Prof. E. Forbes on the Sertularian Zoophytes. 



new arrangement of the Sertularian polypes is necessary, involving 

 the dismemberment of the genera Sertularia and Plumularia. 



A word on the classification of Zoophytes in general. The 

 great groups of this class as at present constituted are not of 

 equal value, which they should be if the acknowledged arrange- 

 ments were strictly natural. This will be found on inquiry to 

 arise from the including of the Bryozoa among Zoophytes proper, 

 and the merging of the Hydraidce, Tubulariadce and Sertulariadce 

 in one order. The anatomical structure of the Ascidioida or 

 Bryozoa removes them altogether from the class of Zoophyta into 

 that of Mollmca, where they should form an order of Mollusca 

 Tunicata, parallel with the group of compound Tunicata of which 

 Botryllus and such forms are examples. The Zoophyta proper 

 may be divided into four very natural orders, the most prominent 

 characters of which will be found in the arrangements of their 

 reproductive system : — viz. 1st, Those which present the ovaries 

 in the form of external bud-like bodies, including the Hydraida 

 and Tubular iadae. 2nd, Those which have the ovaries formed out 

 of transformed branches or pinnae, as the Sertulariadce. 3rd, Those 

 which have the ovaries included in the substance of the polypi- 

 dom, as the Asteroida ; and 4th, Those which have the ovaries 

 forming a part of the internal constitution of the individual po- 

 lypes, as the Zoophyta Helianthoida. 



These four orders I regard as natural, and therefore equal in 

 value. 



This paper must be understood only as a sketch ; circumstances 

 having prevented my working out the inquiries upon which it is 

 founded. But though the data are not numerous, I conceive 

 they are sufficient to warrant my broaching the idea of the mor- 

 phology of the polype-vesicle as presented to the Section at this 

 Meeting. That idea is the clue by which we may be led to more 

 perfect researches on this interesting and important subject, and 

 such naturalists as are inclined to admit its truth must also see 

 that it suggests new questions in the philosophy of zoology. 



EXPLANATION OF PLATE X. 



I. Vesicles of the first order. Fig. 1. Branch and vesicle of & Plumularia 

 from Algoa bay (a, the vesicle; b, the branch). In this case the vesicle re- 

 tains the dimensions of the branch, and is formed by the inflexion and union 

 of the pinnae and polype-cells. Fig. 2. Vesicle of another species of Plu- 

 mularia, in which the axis is much shortened, while the pinnae are all pre- 

 sent. Fig. 3. Back of the same, showing the midrib and the manner in 

 which the ribs of the vesicle spring from it, for comparison with^. 4, re- 

 presenting part of a branch of the fame species with its pinnae and midrib. 

 It is evident that the midrib of the vesicle is identical with the midrib of a 

 branch, and that the denticulations of its lateral ribs correspond to the su- 

 perior elongated teeth of the polype-cells of the pinnae. 



II. Vesicle of the second order. Fig. 5. Portion of a branch of Thoa 

 muricata, showing the arrangement and form of the polype-cells. Fig. 6. 



