424 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
(1) mandible not deeply divided, and possessed of a palp ; (2) proximal 
lobe of second maxilla not reduced ; (3) second maxilliped, with its 
terminal joints not expanded nor distorted as in tlie typical Caridea; 
(4) exopods present on certain thoracic feet. 
Annulata. 
Structure of Lug-Worm.* — Messrs. F. W. Gamble and J. H. Ash- 
worth have studied Arenicola marina , and have made out the following 
(among other) new points. On the Lancashire coast, and probably else- 
where, two well-marked varieties of Arenicola marina occur, differing in 
general appearauce, habits, structure of gills, and periods of maturity. 
The cilia lining the gastric region of the gut are specially arranged 
(1) on the sides of a ventral groove which is continued to the anus, and 
(2) on curved shallow grooves running downwards and backwards into 
the former. The currents carry a stream of mucus and digested food 
backwards away from the mass of sand in the gut. The ventral groove 
is morphologically equivalent to the similar structure of Oligognathus r 
and probably to the “ siphon ” of Capitellids. 
Each of the two hearts contains a cardiac body, composed of masses 
of granular and vacuolated cells, projecting into the ventricle. Func- 
tionally they may be regarded as glandular valves preventing reflux 
into the gastric sinuses. 
Both the large pinnately-branching, and the smaller dendritic, types 
of gill occur in A. marina. A lapsus pennse will be noticed in the de- 
scription of the efferent branchial vessels. 
The brain is divided by a narrow cleft throughout the greater part 
of its length. The anterior cornua supply the prostomium and the 
buccal papillae, and give off the oesophageal nerve-connectives. The 
middle region of the brain supplies the upper part of the prostomium, 
and the posterior cornua innervate the nuchal organ. 
The nuchal organ, though apparently single, shows traces of a double 
origin. It is probably an olfactory organ, and is developed from the 
posterior region of the prostomium. The otoliths consist of quartz 
grains surrounded by a delicate chitinoid film, as Ehlers stated. 
The first pair of nephridia are in process of reduction. In adult- 
examples, the terminal portions of the nephridia act as receptacles for 
the ripe ova or spermatozoa. 
“ The general analogies of Arenicola with certain other limnivorous 
Chaetopods are very startling. With the Sipunculids the Arenicolidae 
agree in the chitinous spines tipping the proboscis papillae, the buccal 
papillae, the strong retractors of the ‘ proboscis,’ the capacious and 
largely unsegmented coelom, the general character of the musculature, 
the thin-walled looped alimentary canal witli its ciliated ventral groove, 
the action of the body-wall in producing waves of coelomic fluid auxiliary 
to the process of burrowing and defaecation, and lastly, the pigmented 
nuchal organ. If we acknowledge the many points of agreement, which 
have for the most part arisen independently, between these two distantly 
related families under similar conditions of life, the true relationship 
between Arenicola and other genera of Polychaets can only be ascertained 
* Quart. Journ. Micr. Sci., xli. (1898) pp. 1-42 (5 pis.). 
