288 



ROBERT TRACY JACKSON ON THE 



wii-e and suspended between stakes. The plates Avere placed in jiaii's with the edges 

 apposed so that the centres Avere widely separated and chambered pockets were thus 

 formed. They were fairly successful. In 1888 when tlie pond was continually full the 

 plates worked moi-e successfully, the oysters growing with extreme rapidity. The 

 rapid growth was pro])al)ly due to the fact that the oysters were suspended and there- 

 fore kejrt very clean and also to the increased warmth of the water in the shallow pond 

 as compared with water in the open bay. In the chambers formed by the apposed plates, 

 similar forms to those found in the drain ])ipes were attached or entrapped. The value 

 of obtaining fixed forms like oysters on thin wood is that they are easily removed for 

 study or may be decalcified and sectioned without removal. Should wooden plates be 

 desired, it is best to secui'e those which are cut from the block in a dish-form, not those 

 which are cut flat and then pressed into a dish-form; because the latter flatten out when 

 exposed in water, whereas the former retain their curved shape. 



Cocaine was found a most valuable reagent in paralyzing Pelecypods which it was 

 Avished to kill distended or to dissect alive. A four per cent solution was used and was 

 added gradually to a small volume of sea-water containing the animal. By this means 

 only Avas I successful in killing young Pectens with the byssal attachment intact. Ad- 

 ditional material was collected at various localities near Boston and some Avas loaned me 

 by museums as noted in the text. 



III. Classification or Stages of Gkoavth and Decline. 



In successive stages of groAvth of the individual, peculiarities ai'e often found which 

 are characteristic of definite periods. These ])eriodical characters have been studied, 

 especially in their later phases, by Professor Hj^att in fossil Cephalopods, and as a con- 

 venience in considering the several periods, he has (33-4) devised a nomenclature of the 

 stages of growth and decline,^ which he believes is applicable throughout the animal 

 kingdom.^ 



We Avill now consider this classification in its application to the mollusca. The no- 

 menclature dcAdsed for early embryonic stages consists of names very broadly recalling 

 the grand divisions of the animal kingdom. They are jjrotembryo, mesembrj'o and met- 

 embryo. The foUoAving names, neoembryo and typeinbryo, are applied to later stages 

 of embi-yonic de\'elopment. 



Professor Hyatt considers that the neoembryo mollusc is the trochosphere stage, and 

 that it has not yet acquired characters peculiar to a mollusc. The trochosphere stage 

 (PI. XXIII, figs. 3-4), before the formation of the shell-gland, certainly has not yet ac- 

 quired characters Avhich Avould define it as a mollusc, and Ave would limit the neoembryo 

 stage to that period of development.^ A comparison of this stage in molluscs Avith Pro- 



' Besides the references cited, this paper was published 

 in the American Naturalist October, 1888. An al)stract 

 was published in Science, A''ol. xi, No. 260. 



- Professor Hyatt says that his nomenclature is similar 

 to that adopted by .Professor Haeckel ; but is also supple- 

 mentary, aud based on morphological, rather than on phys- 

 iological grounds. In his paper he discusses the relations 

 of the two systems of nomenclature. 



^ The ideal trochosphere of Professor Lankester (41) as 

 that author says : "... exactly agrees with the lar- 

 val form of many Ch.Ttopod worms and other Cffilomata ; 

 most remarkable is its agreement with the adult form of 

 the wheel animalcules or Rotifera . . . ." Again he 

 says : "So far the young mollusc has not reached a defi- 

 nitely mollnscan stage of development, being only in a con- 

 dition common to it aud 'other Ccelomata.' " Some of 



