Along the margins of these shallow rock-pools with sandy bottoms, we
see many little grey shrimp-like creatures, with thick-set arching
bodies, which swim rapidly, and with much vibration of the abdominal
feet, from weed to weed. If we catch one and lay it on the rock or on
the wet sand, it appears very helpless, for its vertically thin shape
and arched attitude preclude the possibility of its crawling: it falls
upon its side, and vainly struggles round and round as on a pivot, or
makes aimless jerks by throwing out the tail. It is of a greenish blue,
or glaucous colour, marked with red dots. This is the common Locust
Screw,[75] which is so confined to sea-water, that it has been affirmed
that if put into fresh water it presently dies. But the experiments of
Mr. Robertson negative this statement:[76] he has found that five out
of seven, after being eighteen hours in rain-water, continued to live
upon being returned to salt water.
I delight to trace the manifestations of parental love in these tiny
forms of animal life. It had long been known that the females of this
species display a solicitude for their infant offspring, which they
carry for some time about with them; but the following very pleasing
details of maternal manners have recently been observed by my relative,
Dr. James Salter, and by him communicated to the learned zoologists who
are now engaged upon the history of this order:--
“On catching a female with live larvæ, nothing is seen of the progeny
till the parent has become at home in the aquarium, when the little
creatures leave her, and swim about in her immediate neighbourhood. The
plan I have adopted to watch this curious habit of maternal protection
has been to place a single individual in a bottle of sea-water. After
a time, and that soon, the little crustacean seems at ease, and swims
slowly about, when the young fry leave, and swarm around her in a
perfect cloud; they never leave for more than half or three-quarters of
an inch, and as she slowly moves about they accompany her. If, now, one
taps the sides of the bottle with one’s finger-nail, the swarm of larvæ
rush under their parent, and in a second are out of sight. The parent
now becomes excited, and swims about quickly, as if trying to escape;
but on letting the bottle containing her rest quite still on a table,
she soon gets composed, when out come the young larvæ again, and swim
about as before. This may be repeated as often as the observer wishes,
and always with the same result. I have only seen this in one species,
but it is quite a common species in Poole Harbour, and I have watched
the interesting habit many times.”[77]