Development and Biology of the Larva of Saccoglossus Horsti (Enteropneusta)

Abstract

Larvae of Saccoglossus horsti were reared in the laboratory, and their developmental history from the egg to the five gill-slit stage studied. The immature eggs varied from 0.23 to 0.30 mm in length and from 0.15 to 0.22 mm in breadth. They were irregular, opaque, finely granular and creamish grey in colour. They became spherical on maturing. Fertilization resulted in the rapid erection of a fertilization membrane, making the eggs buoyant. Two similar polar bodies were extruded shortly afterwards, marking the plane of the first cleavage which, with the second, was holoblastic and meridional. Subsequent cleavages were different in the animal and vegetative tiers. There was evidence of radial cleavage during the 16- to 32-cell stage. A hollow blastula was formed at the 9th to 10th cleavage stage, and gastrulation by invagination followed. The blastocoele was completely obliterated and a typical archigastrula resulted. This rapidly became uniformly ciliated and developed a telotroch around the closing blastopore. The component cilia of the telotroch imparted a slow rotatory movement to the embryo. Axial elongation and the growth of an apical tuft were accompanied by the formation of a faint annular groove. This groove marked off the definitive proboscis and the anterior part of the collar. Hatching followed 30 to 36 h after fertilization, and the larva became planktonic. During its lecithotrophic existence the larva developed a second annular groove anterior to the first, marking off the definitive proboscis from the anterior region of the collar. No definite phototaxis was detectable. Swimming movements were spasmodic. The larva rotated in a clockwise direction when viewed from the apical tuft. The spiral mode of propulsion and the propelling action of the telotroch is discussed. Settlement occurred some 2 days after hatching. A post-telotrochal adhesive patch was developed just prior to settlement, enabling the larva to adhere tenaciously to the substratum. After settlement further elongation of the main axis occurred, a well-defined proboscis, collar and trunk were rapidly differentiated. Of particular interest is the development of a long, muscular strongly ciliated post-anal tail. A dispersal period of about 6 1 2 to 7 days occurred prior to settlement. The existence of this phase prior to the animal adopting the adult mode of life demands that the mode of development of certain members of the family Harrimanidae be regarded as indirect and comparable in many respects to that known for some of the family Ptychoderidae. The mouth, anus and gill apertures became functional at much the same period, viz., at the onset of the burrowing phase. Remarkable growth movements initiated during the late planktonic phase were accelerated after settlement. This resulted in the translation of the telotroch to a latero-ventral position on the trunk and tail. The behaviour of the tail during the process of ciliary feeding, as well as during the coursing through the burrow, was observed. Ciliary reversal occurred on collar, trunk and tail. This phenomenon is discussed. Special tactile cilia have been described. They occurred on the dorsal and latero-dorsal surfaces of the trunk and tail. There was some evidence of gregariousness. The possibility of this larval habit is briefly considered in relation to the dispersal of the adults in the field. The homologies of the Enteropneusta and the Pterobranchia are discussed in some detail, with particular reference to the tail of the larval Saccoglossus horsti , and the stalk of the genus Cephalodiscus .