ADVERTISEMENT. vii 



Actinoplirys (1863), he discovered that 'all vihratile cilia originate in tlie amor- 

 phous intercellular substance,' and do not form direct prolongations of cells. In 

 1864, appeared a brief paper, in which he showed that Tubularia was not parthen- 

 ogenous, having found, by the aid of Tolles' improved quarter of an inch objec- 

 tives, that it produced eggs. Perhaps the most important work he has done is in 

 his studies on the affinities of the sponges. In Novembsi*, 1866, appeared, in the 

 American Journal of Science and Arts, a brief paper, entitled ' Conclusive Proofs 

 of the Animality of the Ciliate Sponges, and of their Affinities with the Infusoria 

 Flairellata.' While he had in his Lowell Lectures endeavored to show that there 

 was a unity of plan in the organization of the Protozoa, their bodies being arranged 

 in the form of a helix, he now endeavored to show that the sponge did not depart 

 from the protozoan type. By the discovery of a remarkable form (Codosiga) he 

 was enabled in it to trace a link, in his opinion, uniting the sponges with the 

 flagellate Infusoria, such as Monns, Anthophysa, and Codosiga. In the full 

 memoir, which was published a year after, with numerous figures, under the title 

 'Spongise Ciliata? as Infusoria Flagellata,' he attempted to establish the homology 

 of the flagellate cells, constituting the tissues of the sponge, with the flagellate 

 Infusoria. He demonstrated, by the use of the superior objectives made by Tolles, 

 that these cells are like Monads, with contractile vesicles, nuclei, a collar, and 

 flagellum ; that the sponge was in fact a compound monad, and not a compound 

 amoeba, as insisted on .by Carter in 1854-57, and Ijieberkuhn in 1856 and 1857. 

 This was a great step in advance of previous observers. Certainly an organism 

 with cells so highly difl"erentiated as those in the sponge cannot be a plant, and 

 while, as Clark observes, Carter had ' been the first to present anything like 

 decisive proofs of the animality of the sponges,' yet this was confirmed and 

 demonstrated still more completely by Clark himself. In this memoir he insists 

 upon the fact that these simple ' monas-like infusoria,' making up the compound 

 body of the sponge, were undoubtedly endowed with a distinct mouth, afterwards, 

 in 1871, distinctly seen ; while Carter described them as engulfing food like an 

 amoeba, any part of the cell acting as a mouth. Of course it is necessary for our 

 author to prove that Monas is au animal. This he does conclusively, showing it 

 has a distinct mouth, with a 'li[),'into which food is thrown by tlie flagellum. 

 The cells or zooids of the sponge (Leticosolenia) agree with Monas in all respects, 

 except that he did not detect the mouth, though he saw currents of floating 

 particles which ' are constantly whiided in by the flagella and made to impinge 

 upon the area within the collar.' 



The study of the sponges has since the publication of this important memoir 

 been pursued by Oscar Schmidt, Miklucho ^lacleay, and Ernst Haeckel. Con- 

 siderable advance has been made regarding tlie organization of the adult, while 



