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THE AMERICAN BEE JOURNAL. 



[March 



/. de Salpa, in which he expressed the view that the 

 solitary salpae proceeded from the individuals of the 

 chain and the latter from the solitary ones. Cha- 

 misso's discovery was but little appreciated at first; 

 it was even ridiculed as the vagary of a poet, until 

 it was brilliantly defended by Steenstrup in 1842, 

 and confirmed and expanded later by the accurate 

 investigations of other zoologists. We know now 

 that the loosely connected chain is composed of 

 hermaphrodite sexual animals, generating an em- 

 bryo usually from one egg only, which remains 

 connected for a time with the mother by means of a 

 kind of placenta, and is nourished by it until, hav- 

 ing attained a considerable size, it escapes and 

 forms the solitary or isolated salpa — the only case 

 of viviparity among the tunicata. The solitary 

 salpa then generates a chain of sexually developed 

 individuals by gemmation from buds, which take 

 the place of male and female organs of generation, 

 and thus represent their nurse. 



On the coasts of the North and Baltic Seas im- 

 mense swarms of clear, watery, bell-shaped crea- 

 tures may be perceived in summer, swimming 

 slowly around below the calm surface of the water, 

 with their convex surface upward and their con- 

 cave downward. These are the Aurelia aurita, L., 

 a species of acraspedote, or unfringed medusa, some 

 of which are male and some female, as is the case 

 in all medusae. The sexual organs are ruffle-like 

 folds on the inner skin of four bags or folds in the 

 gastrical cavity, which open outward at the bottom 

 of the stalk. By simple ciliary motion the seed of 

 the male passes into the bags of the female and 

 fecundates the eggs. These then pass out into the 

 folds of the tentacles, where they are developed to 

 embryos, which are provided with a very tender 

 covering of cilia, and move about freely in the 

 water like infusoria. This phase of evolution was 

 formerly considered as a separate species, called 

 planula. Soon, however, the cilia falls off, and the 

 animalcule, thus deprived of its locomotive organs, 

 sinks to the bottom, attaches itself to firm objects, 

 and grows longer. In the free end a cavity soon 

 appears, which gradually increases and is developed 

 into a mouth, from which wart-like excrescences or 

 papilla? shoot out and are afterward converted into 

 tentacles. The animal has now the appearance of 

 a polypus; and it was, indeed, formerly so consid- 

 ered, and called hydra tuba. After some time — 

 .perhaps months — a circular depression is seen just 

 below the crown of tentacles, followed by others 

 behind it. These depressions become deeper and 

 deeper, and short projections appear in their edges, 

 which afterward also develop into tentacles. The 

 whole now bears a distant resemblance to the so- 

 called strobila. or fir cone, or to a set of flat cups 

 resting on a columnar foot, the polypus. The sepa- 

 rate divisions of the strobila are the origin of the 

 future medusae. They develop more and more, one 

 after another, separate from their pedestal, and 

 afterwards attain their permanent form, size, and 

 maturity. They now turn the convex surface by 

 which they were attached, upward, while the 

 mouth, which was before turned up, now points 

 downward. In the aurelia there is, therefore, an 

 intermediate or nurse generation during the poly- 

 pus stage, in which the animal is multiplied in an 

 agamic way by gemmation and fission. Each of the 



individuals so produced is again developed into a 

 sexual medusa. 



In medusae of lower organization belonging to the 

 hydroids, which Gegenbauer has called craspedote, 

 because their disk is provided with a velum, a sim- 

 ilar kind of alternate generation takes place, with 

 the exception, however, that the polypoid nurse 

 reaches a much more advanced stage of independent 

 development after leaving the ovum. It grows to a 

 stalk of considerable size, and puts forth numerous 

 polypus-buds. It is only when the colony has 

 attained a high degree of development that the 

 sexual animals are formed, which separate from 

 the stalk, swim about independently, and deposit 

 their eggs in remote spots. 



In other hydroids the nurse acquires a still 

 greater importance. In them, as in our sweet- 

 water polypi, the sexual progeny appears only in 

 the shape of globular appendages, which are not 

 capable of being evolved into independent animals, 

 but remain attached to the polypus-stalk, and re- 

 semble organs for the production of the sexual 

 secretions. 



We may with Gegenbauer call this latter form of 

 alternate generation imperfect metagenesis. We see 

 another remarkable instance of it in the peculiar 

 many-shaped colonies known as Siphonophoree, 

 which swim about freely in the sea, and of which 

 the vraya dipheys, Blaine, occurring in the Atlantic 

 and the Mediterranean, may serve as an example. 

 From the transparent ovum of this animal a ciliated 

 larva is hatched. The plastic material contained in 

 the body of this larva or nurse is then differentiated 

 into a locomotory piece, (the posterior of the two 

 swimming-bells at the beginning of the stalk of a 

 ripe colony,) and an appendage which afte ward 

 becomes the second bell and the common strlk of 

 the whole colony. The individuals now budaforth 

 from this stalk in a fixed order, but do not separate. 

 They remain so connected that their abdominal 

 cavities all open into the canal passing through the 

 common stalk. These individuals are not by any 

 means formed alike, nor do they serve the same 

 physiological purpose. The principle of the division 

 of labor, which is carried out in the solitary ani- 

 mals so that their organs become constantly more 

 numerous and more perfect, is here applied in such 

 a manner that the various functions of animal life, 

 motion, alimentation, defense, and aggression, as 

 well as sexual reproduction, which is otherwise 

 confined to single individuals, are here distributed 

 among all the animals of the whole colony. In 

 every tuft along the stalk, which sometimes num 

 bers as many as fifty of them, we distinguish 

 nourishers in the form of trumpet-shaped append- 

 ages with orifices called suction-tubes ; aggressors, 

 in the form of long contractile filaments or tentacles 

 furnished with microscopic weapons (nettle-cells) 

 at their knobs ; defenders, in the form of stiff scales 

 or helmets attached to the nourishers for purposes 

 of defense ; reproducers, developed after all the rest, 

 in the form of racemous dioecious capsules swinging 

 in small (special) swimming-bells. By the alternate 

 contraction and expansion of the bell-shaped swim- 

 mers at the upper end of the colony, (the base,) with 

 which the smaller special swimming-bells move in 

 time, the whole colony is propelled through the 

 water. 



