152 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. X, NO. 5. 



aperture, and. as the cloaca of each ascidiozooid opens into the common cloaca, this becomes filled 

 with water, which, overflowing through the open end, tends to drive the colony through the 

 water in the opposite direction. This current is modified and controlled bv the diaphragm, and 

 by the movements of the ascidiozooids, so that the progress through the water is not slow and 

 continuous, but rapid and intermittent. A muscle, shown near the cloacal aperture in figure 8, 

 extends from each ascidiozooid to those adjacent to it and binds all the units of the colony 

 together, so that they are able to make concerted movements, and thus to control the expulsion 

 of the locomotor current. 



The structure and the development of Pyrosoma have been well described by many natural- 

 ists, and a good account of the more essential features is to be found in the text-books. Those 

 who wish for a more complete account should consult the memoirs of Herdman (Report on 

 the Tunicata collected during the voyage of H. M. S. "Challenger," XXVI-XXVIII), of 

 Salensky (Beitrage zur Embryonalentwicklung der Pyrosomen. Zool. Jahrb. IV and V), and of 

 Seeliger (Zur Entwicklungsgeschiehte der Pyrosomen. Jenaische Zeitschr. fur Wiss. Naturw. 

 XXIII.. and Die Pyrosomen., Leipzig, 1895). 



The subject of this memoir is a new form of Pyrosoma, which seems to me to be generically 

 different from all that have been described; but, as its differences from the forms that are 

 known do not involve its fundamental structure, I have little to add to the published accounts, 

 although a brief sketch of the origin of the colony will be the most satisfactory way to explain 

 the meaning of the terms that are to be used in describing the species. 



The egg of Pyrosoma gives rise to an embryo which, while it acquires some indications of 

 ascidian structure, remains rudimentary and quickly degenerates. It is commonly called by the 

 name Cyathozooid, given to it by Huxley. It is shown, in an advanced stage of degeneration, at 

 ey in figure 1. Before it begins to degenerate, it gives rise to a tubular outgrowth, which becomes 

 divided, by constrictions, into four segments, each of which ultimately becomes an ascidian. 

 The four segments are in a row at first but as they grow and develop into ascidians they twist so 

 as to form a zone or girdle around the rudimentary Cyathozooid, as is shown in figure 1. These 

 four primary ascidiozooids are the foundation of a new colonial Pyrosoma. As figure 1 shows, 

 they are inclosed in a common mantle of cellulose, and are arranged in a circlet around a common 

 cloaca into which the cloaca of each opens, while the mouths are on the outer surface. The sur- 

 face that is below in figure 1 is the closed end of the colonial tube, while the opening is above in 

 the figure, in the center of the rosette of eight tubular processes that are shown in the figure. 

 In all ordinary Pyrosomas, the circlet that is formed by the four primary ascidiozooids is a 

 circle, but in the species that is the subject of this paper it is an ellipse; the ascidiozooid marked 

 /id 1 and its fellow lying in the long axis of the ellipse, and the one marked pa 2 and its fellow 

 in the short axis. They are also shown, at a later stage, at pa 1 and pa 2, in figure 2. 



The four primary ascidiozooids soon begin to multiply by budding, and thus to lay the 

 foundation for a new colony, which may ultimately consist of thousands of ascidiozooids, all 

 produced, either immediately or indirectly, as buds, by the four primary ascidiozooids. As 

 each ascidiozooid that arises as a bud soon begins to produce buds in its turn, only a few 

 of those that enter into the structure of an adult colony arise immediately from the primary 

 ascidiozooids. 



Few young colonies, large enough to have all the characteristics of the full-grown colony, 

 and yet small enough and transparent enough to be studied with a microscope, have been found, 

 and none have been adequately figured. 



Among Doctor Grave's specimens is one, about half an inch long, which he had stained and 

 mounted in balsam. It is shown in figure 2, with one of its flat sides toward the observer. As 

 it is small enough to lie studied as a transparent object under the microscope, and is yet, in all 

 essential particulars, a fully developed Pyrosoma, it presents a more complete picture of the 

 organization of the colony than any drawing that has been published. The colony is represented 

 with its closed end below, and the open end with its diaphragm above. It consists of seventy 

 ascidiozooids, arranged in seven rows or verticils, and of numerous buds. There are four 

 ascidiozooids — the primary ascidiozooids — in the first row, eight in the second, ten in the third, 



