Saccoglossus horstisp.n., an Enteropneust occurring in the Solent

1.S. horstioccurs on the Hampshire coast of the Solent near the mouth of the Lymington River, in glutinous grey mud associated withCorophium volutator. 2. The species is distinguished by the following characters: coloration; dorsal and ventral grooves present on the proboscis throughout its length; collar forming a slight operculum posteriorly; ventral muscle bands of the trunk forming projecting ridges; rounded genital ridges present, the gonads beginning within 1 mm. of the collar; 100–140 pairs of gill slits; longitudinal muscle fibres of the proboscis arranged in nine or more concentric rings; ventral septum of the proboscis short; stomochord straight, with ventral diverticulum and wide lumen throughout; no dorsal glomerulus present; single proboscis pore on the left side; proboscis skeleton embracing half to two-thirds of the circumference of the buccal cavity and extending one-fourth to one-third of the length of the collar; four epidermal zones present in the collar; collar cavities completely separated; anterior extensions of the collar cavities extending into the neck of the proboscis; perihaemal cavities separate posteriorly but confluent in front of the level of the tips of the crura, and extending into the neck of the proboscis as far as the proboscis pore; neural keel well developed and in continuity with the epidermal nerve layer at intervals; no dorsal diverticulum of the buccal cavity; branchial epidermis characterized by the predominant intensely eosinophil glandular elements; tongues broader and projecting further into the lumen of the pharynx than the septa; epibranchial ridge formed by two convex ridges bounding a median groove; oesophagus divided into three regions; 4–8 intestinal pores; hepatic region of gut not sacculated and with an expanded lumen; gonads not lobed; oocytes attain a size of 0·23 × 0·17 mm.; yolk cells present in the ovaries. 3. The probable identity of the specimen ofSaccoglossusfrom the French coast of the English Channel, recorded by Caullery & Mesnil (1916) asS. kowalevskyi, with this species is discussed.