424 MEMOIRS OF THE AMERICAN ACADEMY. 



system are developed from the digestive as diverticula. So that the water-system 

 of Tornaria, in spite of its dorsal pore, can in no way be homologized with the water- 

 system of Echinoderms, even if, in addition to the different mode of its development, 

 the presence of a heart and of a muscular band supporting it did not show that we 

 had to deal with an organ which has no homologue in any of the numerous Echino- 

 dermoid larvae thus far observed. 



Ah far as we know the Embryology of the Planarians, of which the development 

 is somewhat analogous to that of Echinoderms, as known from the observations of 

 Muller and Metschnikoff, we still have between the two modes of development radical 

 differences. In all Echinoderms, without exception, the young Echinoderm is devel- 

 oped upon the water-system of the Pluteus as a bud, as it were, and gradually en- 

 croaches upon the scaffolding which has supported it, and finally resorbs the whole 

 Pluteus within itself. No such mode of transformation exists in any known Planarian 

 or other AnDelid larva. In this group we find without exception that the transfer- 

 mations consist of a very gradual change of one stage into another, and that by an 

 elongation or contraction of the different parts of the larva at its various stages of 

 growth, and by a gradual modification of the topography of the organs, the larva 

 passes little by little into its adult stage, — a condition of things entirely fulfilled by 

 the development of Tornaria into Balanoglossus, which, as far as the last phase of its 

 growth, the Balanoglossus, certainly shows nothing bearing any affinity to the Echi- 

 nodermoid mode of budding upon the water-system of the Pluteus. 



We possess fortunately an admirable anatomy of Balanoglossus by Kowalevsky, 1 

 who has rediscovered, as it were, the Balanoglossus first figured by Delle Chiaje, 2 

 and about which nothing of any value, except the short notices of Keferstein 8 and 

 of Quatrefages, 4 had been written since Delle Chiaje's time. I shall therefore, in 

 referring to Kowalevsky 's memoir, be able, from the study of the young immediately 

 following the Tornaria stage and of younger specimens of Balanoglossus than those 

 Kowalevsky has investigated, to give an explanation of the nature and function of 

 many of the organs of this interesting animal which could not be explained merely 

 from the study of the adult. 



As is well known from Muller's figures and descriptions of the 

 of Tornaria, the large circular belt of vibratile cilia is only developed in the older 



y° 



1 Kowalevsky. Anatomie du Balanoglossus, Mem. Acad. St Pet., 1866, X. No. 3. 

 * Delle Chiaje. Mem. sull. Stor. e Not. degl. Anim. s. Verteb., PI. LVL, f. 36. 

 1 Keferstein. Zeitschrift f. Wiss. Zool., 1863, XII. 91. 

 4 Quatrefages. Ann. Sc. Nat., 1846, VI. p. 184. 



