The life‐history of Clymenella torquata (Leidy). (Polychaeta.)

Abstract

SUMMARY.The spawning period of Clymenella torquata (Leidy) on the flats at Whitstable is a sharply defined one lasting for two or three days at the time of spring tides near to the middle of May.It is suggested that the immediate stimulus for the worms to spawn is not a temperature effect but is in some way related to the tides, possibly being due to the extra time of exposure at low water springs.The growth, maturation and structure of the germ cells is briefly described.The sex ratio in the worms is 1.5 males to 1 female.Fertilization and cleavage are briefly described, but no important deviations from the usual spiral cleavage pattern were noted.The young larva, which is of the demersal type, is spherical and uniformily ciliated. It is very opaque and contains numerous oil globules derived from the yolky oocyte.At about twenty‐four hours after fertilization the primordium of the trunk becomesapparent asalobe at the hinder end and da gapappearsin the ciliation of the dorsal surface, so separating a “prototroch” form a telotroch which, however, remain connected by a neurotroch. An apical tuft and a pair of brown eye spots appear at the anterior end. From the 24‐hour stage onwards the larval trunk grows rapidly and the larva becomes elongated and cylindrical. By five days after fertilization the first three larvalsegments have been demarcated. The second of these bears on each side a pair of chaetae. The dorsal chaeta in each pair is a simple bristle and the ventral one, a small uncinus. The first segment. which is the forerunner of the peristomium, is always achaetous whilst the third has not yet acquired its chaetae.More segments with a similar arrangement of chaetae are gradually added as the larva elongatess. Sticky from the first, the larvae now secrete quaantities of viseid mucus and normally are adherent to the substratum. The ciliation from this time onwards becomes progressively reduced.Seven days after fertilization the larvae have reached a length of 0–6 mm. Rudiments of the notopodia are present and al powers to swim by means of cilia are lost. The larvae can move their chaetae and can wriggle within a layer of mucus which completely ensheathes the body. A small coelom now separates the gut from the body wall throughout the greater part of its length and a stomodeal pit has appeared on the ventral part of the peristomium. The gut is still filled with oil globules.In the 10‐day larva the stomodeum opens into the pharynx and the proctodeum opens into the hind gut. The remaining oil globules diminish rapidly from this time onwards and at the 6‐chaetiger stage the larvae begin to feed on bottom‐dwelling diatoms which are swept into the gut by the cilia of the buccal capsule and pharynx. The prostomium, now a distinct conical structure, has lost the apical tuft but bears two pairs of eye spots. Of the ciliated bands, only a reduced neurotroch remains. Small particles from the substratum are incorporated into the mucus sheath around the 6‐chactiger larva to form a distinct tube. Experiments suggest that Clymenella larvae will undergo their normal slow metamorphosis and construct tubes (if only of hardened mucus) on any type of substratum.Growth in size and the addition of more segments continued fairly steadily as the larvae fed within their tubes. On the sixteenth day larvae had acquired seven chaetigers, but after this the stage of development reached by the larvae became rather variable. By the twenty‐third day a few had reached the 10‐chaetiger stage and more bristles had been added to the anterior notopodia. A muscular rim had appeared around the peristomium and an anal funnel with papillae, the forerunners of the cirri of the adult pygidium, had developed. The main regions of the gut, namely, buccal cavity, pharynx, oesophagus, stomach and intestine were also recognizable. Dorsal and ventral vessels containing blood, as yet colourless, could be seen.Between the twenty‐fourth and forty‐fifth day the larvae grew into small worms which could easily be recognized as young maldanids, the full adult complement of twenty‐two segments having been differentiated by 26th June.It is pointed out that Clymenella torquata affords yet another example of a polychaete having large‐yolked eggs and larvae which are bottom‐dwelling from the very beginning of development and must be added to the list of polychaetes which omit the trochophore stage from their life‐history. The influence of this type of developmen on the dispersion of the species is briefly considered.